van't Hoff-Le Bel Centennial - ACS Publications

2 Arrangement and Structure—A Distinction and a Downloaded by UCSF LIB CKM RSCS MGMT on November 24, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0012.ch002

Difference TREVOR H. LEVERE Institute for the History and Philosophy of Science and Technology, University of Toronto, Toronto, Ontario, Canada M5S 1A1 At t h i s celebration of the hundredth anniversary of two seminal papers by the founders of stereochemis t r y , Jacobus Henricus van't H o f f and Joseph A c h i l l e Le Bel , I want to ask why views l i k e those expressed i n t h e i r papers were so long delayed, o r , more fruitfully, why previous chemists, possessing most of the fragments which would be used to create the concepts of stereochemistry, came nowhere near the achievements of 1874. They d i d not try and fail, but instead they d e l i b e r a t e l y avoided concepts which might now seem akin to stereochemistry as one would avoid deserts, bogs, and other dangerous and misleading regions. Let me state the problem more fully. Stereochemistry involves what van't Hoff i n the title of the f i r s t p u b l i c a t i o n of his paper c a l l e d 'the extension into space of the s t r u c t u r a l formulae at present used i n chemistry'. Note i n passing something to which we s h a l l have to r e t u r n : most apparently s t r u c t u r a l formulae p r i o r to the mid-l870s were not intended to express the s p a t i a l arrangement of atoms. This i s implied i n van't H o f f ' s t i t l e , and i s confirmed time and again by h i s predecessors . Kekulé, who appears to have used s t r u c t u r a l formulae i f anyone i n the 1860s did so, was at pains to explain that his r a t i o n a l formulae were reaction formulae*. They were i n i t i a l l y and p r i m a r i l y c l a s s i f i c a t o r y i n function . Kekulé's formulae, however, i n common with almost everyone e l s e ' s , except Benjamin C o l l i n s Brodie's i n his calculus of chemical o p e r a t i o n s , presented molecules as being made up of atoms, as for example i n t h i s representation of a c e t i c a c i d : 1

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F o r a l l t h e p e r e n n i a l doubts t h r o u g h t h e n i n e t e e n t h c e n t u r y about t h e e x i s t e n c e o f a t o m s , almost a l l c h e m i s t s r e c o g n i z e d what J.B. S t a l l o i n 1 8 8 1 c a l l e d t h e m e r i t s o f the a t o m i c h y p o t h e s i s as a g r a p h i c o r e x p o s i t o r y d e v i c e . He r i g h t l y went on t o remark t h a t I t i s a f a c t beyond d i s p u t e t h a t c h e m i s t r y owes a g r e a t p a r t o f i t s p r a c t i c a l advance t o [ t h e use o f t h e atomic h y p o t h e s i s ] , and t h a t t h e s t r u c t u r a l f o r m u l a e founded upon i t have e n a b l e d t h e c h e m i s t , not merely t o t r a c e t h e c o n n e c t i o n and m u t u a l dependence o f t h e v a r i o u s s t a g e s i n t h e metamorphosis o f "elements" and "compounds", s o c a l l e d , but i n many c a s e s (such as t h a t o f the h y d r o c a r b o n s e r i e s i n o r g a n i c c h e m i s t r y ) s u c c e s s f u l l y t o a n t i c i p a t e the r e s u l t s o f experimental research' . So s t e r e o c h e m i s t r y i n v o l v e s t h e a t o m i c t h e o r y and t h e concept o f s t r u c t u r e where t h i s means t h e a r r a n g e ment o f atoms i n space. I t i s w e l l enough known t h a t t h e c o n c e p t s o f atomism and o f s o m e t h i n g l i k e s t r u c t u r e were i m p l i e d s i m u l t a n e o u s l y i n c h e m i c a l l i t e r a t u r e i n 1 8 0 8 i n John D a l t o n ' s A New System of Chemic a l P h i l o s o p h y . The l a s t p a r t o f t h e volume p r e s e n t s t h e a t o m i c t h e o r y , and ends by s t a t i n g t h a t 'when t h r e e o r more p a r t i c l e s o f e l a s t i c f l u i d s a r e combined t o g e t h e r i n t o one, i t i s t o be supposed t h a t t h e p a r t i c l e s o f t h e same k i n d r e p e l each o t h e r , and t h e r e fore take t h e i r s t a t i o n s a c c o r d i n g l y ' . The consequence was t h a t he r e p r e s e n t e d 'An atom o f s u l p h u r i c a c i d , 1 s u l p h u r + 3 oxygen' by t h e c h a r a c t e r s 8

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Of course t h i s i s a v e r y l i m i t e d n o t i o n o f s t r u c t u r e , but o t h e r s were advanced w h i c h were more s o p h i s t i c a t e d . W o l l a s t o n , f o r example, p u b l i s h e d a p a p e r i n l 8 0 8 i n w h i c h he a p p l i e d D a l t o n ' s law o f m u l t i p l e p r o p o r t i o n s t o s u p e r - a c i d and s u b a c i d s a l t s . He found an unexpl a i n e d gap i n t h e p o t a s s i u m o x a l a t e s , and argued t h a t an u n d e r s t a n d i n g o f t h e i n t e r a c t i o n o f e l e m e n t a r y atoms would r e q u i r e not o n l y a r i t h m e t i c a l laws but a l s o a g e o m e t r i c a l c o n c e p t i o n o f t h e r e l a t i v e arrangement o f atoms i n t h r e e d i m e n s i o n s . Examples o f such s p e c u l a t i o n s c o u l d be m u l t i p l i e d . Now i f s t e r e o c h e m i s t r y i n v o l v e s the- i d e a s o f atoms and t h e i r arrangement, and 1 2

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i f t h e s e i d e a s were b r o a d l y f a m i l i a r t o c h e m i s t s from 1808, t h e n c r i t i c a l i n g r e d i e n t s were a t hand. Why were v a n t H o f f and Le B e l not a n t i c i p a t e d l o n g b e f o r e 1 8 7 * ? The answer i s not s i m p l e , and I s h a l l m e r e l y suggest some o f i t s components. Of c o u r s e my s t r a t e g y w i l l be to argue t h a t awareness o f t h e i d e a s o f atoms and t h e i r arrangement was a n e c e s s a r y but not a s u f f i c i e n t c o n d i t i o n f o r the f o r m u l a t i o n o f s t e r e o c h e m i c a l t h e o r i e s , and t o t r y t o probe t h e a r e a o f i n s u f f i c i e n c y . I f one assumes t h a t atomism o f f e r e d a t l e a s t a u s e f u l hypot h e s i s t o most o f t h e community o f c h e m i s t s a f t e r I 8 O 8 , t h e n c o n c e n t r a t i o n on t h e concept o f arrangement s h o u l d best i l l u m i n a t e the area of i n s u f f i c i e n c y . Easy a c c e s s to t h i s a r e a i s p r o v i d e d by one o f t h e n i n e t e e n t h cent u r y ' s most b r i l l i a n t c h e m i s t s , Auguste L a u r e n t , i n t h e p r e f a c e t o h i s Chemical Method o f 1 8 5 4 * . Laurent t h e r e i d e n t i f i e d two s t u m b l i n g b l o c k s i n t h e way o f e s t a b l i s h i n g a c h e m i c a l system — f a c t s , and t h e i r causes. I f one r e s t r i c t e d o n e s e l f t o f a c t s , t h e n no t r u e system would be p o s s i b l e , and one c o u l d a c h i e v e only a d e s c r i p t i v e n a t u r a l h i s t o r y of chemical species organized according to a natural c l a s s i f i c a t i o n . A l t h o u g h L a u r e n t d i d n o t say so a t t h i s p o i n t , he c l e a r l y b e l i e v e d t h a t n a t u r a l c l a s s i f i c a t i o n s were s t e r i l e , and a d v o c a t e d an a r t i f i c i a l s y s t e m . Mere f a c t s were i n a d e q u a t e f o r c h e m i c a l s c i e n c e . What o f the other stumbling b l o c k ? Laurent wrote: 'By s e t t i n g out from t h e i d e a o f c a u s a l i t y , o r o f atoms and t h e i r arrangement, we must d e t e r m i n e i n any p a r t i c u l a r body, w h i c h o f i t s atoms a r e combined s i m p l y , w h i c h o f them a r e combined i n t i m a t e l y , whether they a r e c o p u l a t e d o r c o n j u g a t e d . . . But i n t e r m e d i a t e between f a c t s and t h e i r c a u s e s , we have g e n e r a l i t i e s and l a w s . Would i t not be p o s s i b l e , by r e l y i n g upon them, t o e s t a b l i s h a method* t h a t i s t o s a y , a system o f f o r m u l a e , a c l a s s i f i c a t i o n and a n o m e n c l a t u r e , h a v i n g the advantage o f systems based upon f a c t s , and o f t h o s e based upon h y p o t h e s e s , but w i t h o u t t h e i r i n c o n v e n i e n c e s . T h i s i s what I have a t t e m p t e d t o do i n t h i s work, by e n d e a v o u r i n g t o r e n d e r i t as much as p o s s i b l e i n d e pendent o f a l l h y p o t h e s e s . Such o f them as a r e t o be met w i t h a r e i s o l a t e d , and may be l e f t e n t i r e l y on one s i d e , w i t h o u t any d e t r i m e n t t o the p r o g r e s s o f t h e work. . . . I do n o t . . . r e j e c t t h e r e s e a r c h a f t e r c a u s e s , a l t h o u g h t h e s e may form perhaps but a p e r p e t u a l m i r a g e , d e s t i n e d t o i m p e l us i n c e s s a n t l y t o an e x p l o r a t i o n o f new c o u n t r i e s . ' L a u r e n t ' s i d e n t i f i c a t i o n o f c a u s a l i t y w i t h 'atoms


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and t h e i r a r r a n g e m e n t was u n f o r t u n a t e f o r arrangement, i m p l y i n g t h a t i t was p r o b a b l y u n d i s c o v e r a b l e . I t c o u l d s e r v e a h e u r i s t i c f u n c t i o n , but t h e s e a r c h f o r t h e a c t u a l arrangement o f atoms i n m o l e c u l e s was l i k e l y t o be l o n g i f n o t e t e r n a l l y f r u s t r a t e d . What L a u r e n t has t o say about arrangement i s t h e r e f o r e not v e r y d i f f e r ent from what Auguste Comte has t o say about c a u s a t i o n i n g e n e r a l — i t s e r v e s i t s p u r p o s e d u r i n g t h e adol e s c e n c e o f a s c i e n c e , but has no p l a c e i n t h e f i n a l positive edifice of laws . Comte was L a u r e n t ' s contemporary. I have no grounds f o r c l a i m i n g any i n f l u ence o f one on t h e o t h e r , and many o f L a u r e n t ' s s p e c u l a t i o n s went beyond Comte's p o s i t i v i s m , but t h e e x t e n t o f t h e i r common ground i s s t r i k i n g . They b o t h r e c o g n i z e d t h a t t h e i n n e r arrangement o f m o l e c u l e s was unknown, and, i n t h e p r e s e n t s t a t e o f c h e m i s t r y , u n k n o w a b l e , s o t h a t w h a t e v e r c l a s s i f i c a t i o n one adopted c o u l d s e r v e o n l y as a c o n v e n t i o n and a g u i d e . L a u r e n t and Comte a d m i t t e d l y d i f f e r e d i n t h e i r c h o i c e of c l a s s i f i c a t i o n . Comte f e l t t h a t e l e c t r o c h e m i c a l d u a l i s m o f f e r e d t h e most a p p r o p r i a t e g u i d e t o t h e c l a s s i f i c a t i o n o f c h e m i c a l s u b s t a n c e s and t o t h e p r e d i c t i o n o f t h e i r r e a c t i o n s , w h i l e Laurent r e j e c t e d t h e d o c t r i n e s o f B e r z e l i u s and i n s t e a d p r o p o s e d h i s own n u c l e u s t h e o r y . I n n e i t h e r c a s e , however, was method s u p p l a n t e d by o n t o l o g y . G e r h a r d t c o u l d have been s p e a k i n g f o r b o t h o f them when he w r o t e : '...we have a deep c o n v i c t i o n t h a t a l l t h e metamorphoses o f a s i n g l e s u b s t a n c e a r e c o n t r o l l e d by g e n e r a l l a w s , w h i c h we can s c a r c e l y e n v i s a g e t o d a y , but w h i c h t h e u n i t e d e f f o r t s o f c h e m i s t s w i l l c e r t a i n l y succeed i n discovering... We have been t a k e n t o t a s k w i t h a s o r t o f d i s d a i n f o r p e r f o r m i n g c h e m i c a l a l g e b r a ; we a r e g l a d t o a c c e p t t h i s d e s c r i p t i o n , because we b e l i e v e t h a t t h e t r u e p r o g r e s s o f s c i e n c e does not c o n s i s t o f l i m i t l e s s m u l t i p l i c a t i o n s o f f a c t s and e x p e r i m e n t s , but i n e s t a b l i s h i n g a n a l o g i e s , and g e n e r a l i z i n g them by f o r m u l a e , t h u s f i n d i n g t h e laws w h i c h a r e t h e o n l y g u i d e s t o t h e c e r t a i n p r e d i c t i o n o f phenomena.' The concept o f arrangement was u s e f u l — i t p r o vided Laurent w i t h the seminal i d e a behind h i s nucleus t h e o r y * , brought c h e m i s t r y and c r y s t a l l o g r a p h y c l o s e r t o g e t h e r , and u n d e r l i n e d t h e u n i t y o f o r g a n i c and inorganic s u b s t i t u t i o n s . Thus he p r o p o s e d , f o r example, a r e p r e s e n t a t i o n o f l e a d s u l p h a t e i n f e r r e d from c r y s t a l l o g r a p h i c e v i d e n c e , i n w h i c h l e a d o x i d e and s u l p h u r i c a c i d m a i n t a i n e d t h e i r independent e x i s t e n c e w i t h i n an o v e r a l l r e g u l a r s t r u c t u r e . Fisher has i n t e p r e t e d L a u r e n t ' s d e s c r i p t i o n t h r o u g h t h i s


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But i n s p i t e o f such a t t e m p t s , arrangement was f o r L a u r e n t p r i m a r i l y a h e u r i s t i c concept w h i c h p r o v i d e d general i d e a s . As L a u r e n t s t a t e d i n 1 8 4 5 , i t was i m p o s s i b l e t o r e p r e s e n t a t h r e e - d i m e n s i o n a l atomic arrangement by a l i n e a r f o r m u l a . One s h o u l d t h e r e f o r e leave to others the search f o r formulae i n d i c a t i n g arrangement, a t t a c k i n g them as l o n g as they f a i l e d t o f i n d true f o r m u l a e . I n s t e a d , assuming t h a t s u b s t a n c e s w i t h s i m i l a r but i n d i v i d u a l l y unknown arrangements s h o u l d have s i m i l a r p r o p e r t i e s , one c o u l d c r e a t e a system o f s y n o p t i c f o r m u l a e , e x p r e s s i n g analogies. As f o r arrangement, L a u r e n t c o n c l u d e d , ' S h a l l we always be i g n o r a n t o f [ i t ] ? Who k n o w s ? ' A knowledge o f t h e d e t a i l e d arrangement o f atoms i n m o l e c u l e s was thus a t l e a s t t e m p o r a r i l y r u l e d out by p o s i t i v i s t a t t i t u d e s towards s c i e n c e among p r e c i s e l y t h o s e c h e m i s t s whose thought might o t h e r w i s e have been i n c l i n e d t o c o n s i d e r arrangement. The same a t t i t u d e s , r e i n f o r c e d by p o l e m i c a l b i a s , r u l e d out an a s p e c t o f atomism, w h i c h , i n c o r p o r a t e d i n t h e t h e o r y o f v a l e n c y , was t o be e s s e n t i a l f o r t h e development o f s t e r e o c h e m i s t r y . I am r e f e r r i n g t o t h e concept o f a f f i n i t y . Comte r e j e c t e d i t as m e t a p h y s i c a l , and t h e r e f o r e u n s c i e n t i f i c . Laurent admitted t h a t t h e n a t u r e o f atoms, presumably i n c l u d i n g t h e i r a f f i n i t i e s , i n f l u e n c e d m o l e c u l a r p r o p e r t i e s , but s t r e s s e d t h a t arrangement was o f g r e a t e r i m p o r t a n c e t h a n atomic n a t u r e s . The w i d e - r a n g i n g s u c c e s s e s o f the theory of t y p e s , w i t h i t s s t r u c t u r a l conceptual b a s i s , d i s t r a c t e d a t t e n t i o n from t h e concept o f chemic a l a f f i n i t y * , w h i c h had p r o v i d e d t h e e i g h t e e n t h cent u r y w i t h i t s most s u c c e s s f u l c l a s s i f i c a t o r y t o o l and f u r n i s h e d the b a s i s f o r B e r z e l i u s ' s e l e c t r o c h e m i c a l dualism. S i n c e t h e t h e o r i e s o f d u a l i s m and t y p e s were g e n e r a l l y , i f m i s t a k e n l y , seen i n o p p o s i t i o n t o one a n o t h e r , and s i n c e the t h e o r y o f t y p e s was becoming 2 9

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dominant i n m i d - n i n e t e e n t h - c e n t u r y o r g a n i c c h e m i s t r y , a f f i n i t y ceased t o be a prominent t o p i c i n t h a t r e a l m o f c h e m i c a l d i s c o u r s e . Dumas was one o f t h e p r i n c i p a l v i l l a i n s i n t h e temporary demise o f a f f i n i t y , and seems t o have r e c o g n i z e d and e n j o y e d h i s r o l e . I n the 1 8 5 0 s and 1 8 6 0 s s e v e r a l c h e m i s t s , i n c l u d i n g D a u b e n y and W u r t z , c o r r e c t l y emphasized t h i s a s p e c t o f recent chemical h i s t o r y . So f a r I have argued t h a t t h o s e who adopted a u n i t a r y t h e o r y n e g l e c t e d a f f i n i t y and were a g n o s t i c about arrangement. T h e i r n e g l e c t o f t h e former and a g n o s t i c i s m about t h e l a t t e r were n o t o f c o u r s e w h o l l y p o l e mical or philosophical. Crystallographic analogies had l e d t o t h e o r i e s s u g g e s t i n g c o n s t i t u t i o n a l a n a l o g i e s but as Beudant p o i n t e d out i n 1 8 4 3 , c r y s t a l l o g r a p h e r s h a d l e a r n e d c a u t i o n and were p r i m a r i l y conc e r n e d t o e s t a b l i s h a c l a s s i f i c a t i o n based upon t h e e x t e r n a l c h a r a c t e r i s t i c s o f b o d i e s , and not upon i n t e r n a l ones. Then t o o , as competent m i n e r a l o g i s t s l i k e Dana were w e l l a w a r e , i t was a l l v e r y w e l l t o say t h a t i n p r i n c i p l e c h e m i c a l and c r y s t a l l o g r a p h i c c l a s s i f i c a t i o n s s h o u l d a g r e e , w h i l e i n p r a c t i c e i t was h a r d t o see how they d i d so. A t t h e v e r y l e a s t , t h e d o c t r i n e s o f p r e f o r m a t i o n i n c h e m i s t r y and i n c r y s t a l l o g r a p h y a l i k e would need m o d i f i c a t i o n . There were h o p e s , e x p r e s s e d by L a u r e n t * , G e r h a r d t * , Baudrimont * , Kekulé * , and o t h e r s , t h a t c r y s t a l l o g r a p h y would one day r e v e a l t h e r e l a t i v e p o s i t i o n s o f atoms i n compounds — but o f c o u r s e , as Baudrimont ***and Kekulé * s t r e s s e d , t h e s e would be p o s i t i o n s i n u n r e a c t i n g compounds. R e a c t i o n s would change s t r u c t u r e s , and a c c o r d i n g l y i n f o r m a t i o n d e r i v e d from r e a c t i o n s c o u l d not l e a d t o d i r e c t knowledge o f s t r u c t u r e s . Laurent a c c o r d i n g l y used s y n o p t i c f o r m u l a e , e.g. C H C 1 ' * 0 + 0 f o r c h l o r a c e t i c a c i d * , Kekulé used r e a c t i o n f o r m u l a e * and so on, q u i t e p r o p e r l y , s i n c e c h e m i s t s were conc e r n e d w i t h p r e d i c t i n g c h e m i c a l changes and c l a s s i f y i n g c h e m i c a l s p e c i e s . W i l l i a m s o n ' s b r i l l i a n t work on e t h e r i f i c a t i o n showed b o t h the s u c c e s s and the l i m i t a t i o n s o f a dynamic a p p r o a c h , e x p r e s s e d by the t w i n equations : 3 5

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K i n e t i c theory, with i t s associated chemical k i n e t i c s — B e r t h e l o t ' s Mécanique Chimique — may have seemed t o p o i n t t h e same way.


24

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CENTENNIAL

Thus, a l t h o u g h s u b s t i t u t i o n and t h e r e s t seemed t o encourage t h e view t h a t arrangement was t h e p r i n c i p a l d e t e r m i n a n t o f p r o p e r t i e s , awareness o f c h e m i c a l change made knowledge o f arrangement seem u n o b t a i n a b l e B e s i d e s , phenomena o t h e r t h a n s u b s t i t u t i o n were not so e a s i l y handled. F o r example, compounds produced by b u r n i n g diamond and c h a r c o a l r e s p e c t i v e l y i n oxygen were i n d i s t i n g u i s h a b l e * , but t h e ease w i t h w h i c h carbon d i o x i d e was produced was v e r y d i f f e r e n t i n t h e two c a s e s , and even i f one c l a i m e d t h a t c h a r c o a l and diamond were c h e m i c a l l y t h e same s p e c i e s , they were physically distinct. B e r z e l i u s i n 1840 c h a r a c t e r i z e d such v a r i a t i o n s as i n s t a n c e s o f a l l o t r o p y , and went on i n 1845 t o suggest t h a t t h i s was not r a r e , but was p r o b a b l y a g e n e r a l p r o p e r t y o f t h e elements. He s u g g e s t e d t h a t a l l o t r o p e s o f a g i v e n element were d i f f e r ent s t a t e s o f t h a t e l e m e n t , but made no r e f e r e n c e t o arrangement as u n d e r l y i n g t h e d i f f e r e n c e s . L a u r e n t q u i c k l y p r o v i d e d the m i s s i n g s t r u c t u r a l s p e c u l a t i o n s , w r i t i n g t o B e r z e l i u s t h a t t h e r e was a g r e a t analogy between t h e i r r e s p e c t i v e v i e w s . 'The atom, a c c o r d i n g t o i t s d e f i n i t i o n , cannot be m o d i f i e d ; d i f f e r e n t v a r i e t i e s o f a s i m p l e body α, β, γ, can t h e r e f o r e o n l y be d i f f e r e n t groups. T h a t , i f I have u n d e r s t o o d i t a r i g h t , i s t h e consequence o f your h y p o t h e s i s . As I see i t , t h e c h e m i s t s ' o r d i n a r y atom would be a group o f e l e m e n t s , and d i f f e r e n t groups o f t h e s e elements would c o n s t i t u t e the v a r i e t i e s α, β, γ.' Berzelius p r o v i d e d no i l l u s t r a t i o n , and thought such s p e c u l a t i o n h i g h l y p r e m a t u r e : ' I t i s i m p o s s i b l e t o account f o r t h e cause o f t h e d i f f e r e n c e o f s i m p l e b o d i e s , a c c o r d i n g t o t h e i r a l l o t r o p i e s t a t e , even i f we c o u l d d e t e r m i n e t h e p r o p e r t i e s p o s s e s s e d by each s t a t e . Does the d i f f e r ence perhaps r e s i d e i n a p a r t i c u l a r g r o u p i n g o f t h e atoms o f s i m p l e b o d i e s , i n such a way, f o r example, t h a t 2, 3 o r s e v e r a l atoms would combine t o form groups o f atoms w h i c h w o u l d p l a y t h e p a r t o f a s i n g l e atom, as seems t o be t h e case w i t h s u l p h u r ? o r i s an e l e c t r i c p o l a r i t y m o d i f i e d o r f i x e d up t o a c e r t a i n p o i n t . . . ? Our p r e s e n t knowledge does not a l l o w us t o answer t h e s e q u e s t i o n s . ' I t was not even c l e a r t h a t t h e s e d i f f e r e n t s t a t e s s h o u l d be c l a s s i f i e d as r e p r e s e n t i n g t h e same element. B r o d i e , f o r example, a n a l y z e d g r a p h i t e o x i d e , a p p l i e d Dulong and P e t i t ' s l a w , and found t h a t t h e atomic w e i g h t o f g r a p h i t e was 33· He c o n c l u d e d t h a t t h i s form o f carbon ' s h o u l d be c h a r a c t e r i z e d by a name m a r k i n g i t as a d i s t i n c t e l e ment' *. And y e t t h e r e were some who r e g a r d e d i s o merism and a l l o t r o p y as analogous phenomena, b o t h e x p l i c a b l e i n terms o f a r r a n g e m e n t , even w h i l e 1


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i s o m e r i s m i t s e l f remained p e r p l e x i n g t o many. W i l l i a m O d l i n g i n 1864 t o l d t h e c h e m i c a l s e c t i o n o f t h e B r i t i s h A s s o c i a t i o n f o r t h e Advancement o f S c i e n c e t h a t 'Isomerism i s , i n f a c t , t h e c h e m i c a l p r o b l e m o f the d a y . . . I t i s c u r i o u s t o n o t e the o s c i l l a t i o n s o f o p i n i o n i n r e s p e c t t o t h i s s u b j e c t . Twenty y e a r s ago t h e m o l e c u l a r c o n s t i t u t i o n o f b o d i e s was p e r c e i v e d by a s p e c i a l i n s t i n c t , s i m u l t a n e o u s l y w i t h , o r even p r i o r t o , the establishment o f t h e i r molecular weights. Then came an i n t e r v a l o f s c e p t i c i s m , when t h e i n t i m a t e cons t i t u t i o n o f b o d i e s was m a i n t a i n e d t o be not o n l y u n known, but unknowable. Now we have a p e r i o d o f tem p e r a t e r e a c t i o n , not r e c o g n i z i n g t h e d e s i r e d knowledge as u n a t t a i n a b l e , but o n l y as d i f f i c u l t o f a t t a i n m e n t . And i n t h i s , as i n many o t h e r i n s t a n c e s , we f i n d e v i dence o f t h e h e a l t h i e r s t a t e o f mind i n w h i c h , now more perhaps t h a n e v e r , the f i r s t p r i n c i p l e s o f chemi c a l philosophy are explored. Speculation, indeed, i s not l e s s r i f e and s c a r c e l y l e s s esteemed t h a n f o r m e r l y , but i s now seldom o r n e v e r m i s t a k e n f o r a s c e r t a i n e d truth.' Perhaps i t w i l l be a p p r o p r i a t e t o l e a v e the midc e n t u r y p r o p o n e n t s o f t h e o r i e s e m p h a s i z i n g arrangement w i t h t h e r e f l e c t i o n t h a t t r e n d s such as O d l i n g d i s cusses, r e f l e c t i n g the changing s t a t u s o f i m a g i n a t i o n i n s c i e n c e , may f o r a w h i l e be more i m p o r t a n t f o r t h e progress o f science than substantive d i s c o v e r i e s . A f t e r G e r h a r d t came t o P a r i s i n 1838, L i e b i g c o n t i n u e d t o send him a d v i c e , w a r n i n g him t h a t t h e Academy was 'the i m p l a c a b l e a d v e r s a r y o f t h e o r i e s ' . Even Dumas had s u f f e r e d from t h e o r i z i n g , and i f G e r h a r d t wanted t o i n d u l g e h i m s e l f , L i e b i g admonished, 'For the l o v e o f God, don't w r i t e about t h e o r i e s , e x c e p t f o r German journals'.' L a t e r , Kolbe's b l a s t against van't H o f f ' s s t e r e o c h e m i c a l nonsense was t o be c o u n t e r e d by t h e l a t t e r ' s a s s e r t i o n t h a t i m a g i n a t i o n i n the 1870s was not p l a y i n g 'the r o l e t h a t i t i s c a p a b l e o f p l a y i n g ; even today K e p l e r w o u l d have been a b l e t o r a i s e h i m s e l f as h i g h above h i s s u r r o u n d i n g s as i n h i s own time'. So much, f o r t h e t i m e b e i n g , f o r arrangement and s p e c u l a t i o n . What o f e l e c t r o c h e m i c a l d u a l i s m w i t h i t s emphasis upon a t o m i c n a t u r e s ? B r i e f l y , as Dumas w r o t e i n 1 8 4 0 , i t i s a t h e o r y w h i c h seeks i n p r i n c i p l e t o d e r i v e t h e f u n d a m e n t a l p r o p e r t i e s o f b o d i e s from t h e nature o f t h e i r elementary p a r t i c l e s . Within i t , arrangement i s t h e r e f o r e s e c o n d a r y . I t s emphasis upon a t o m i c p o l a r i t i e s , and i t s c o r r e s p o n d i n g b i n a r y c l a s s i f i c a t i o n suggest a s e r i e s o f p o l a r l i n k a g e s , so t h a t i f one t h i n k s o f arrangement one i s l i k e l y t o do s o i n 5 β

5 7

5 8

5 9

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6 1


26

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terms o f o r d e r , l i n k s i n a c h a i n , r a t h e r t h a n a r r a n g e ment, b r i c k s i n a t h r e e - d i m e n s i o n a l e d i f i c e . Berzeliu s , a r c h i t e c t and champion o f e l e c t r o c h e m i c a l d u a l i s m , was w i l l i n g t o c o n s i d e r more n e a r l y s t r u c t u r a l n o t i o n s . On one o c c a s i o n he even went so f a r as t o propose t h a t o r g a n i c atoms had a c e r t a i n m e c h a n i c a l s t r u c t u r e , w h i c h e n a b l e s us t o d e p r i v e some o f them o f c e r t a i n e l e m e n t a r y atoms w i t h o u t a l t e r i n g the whole v e r y much'. Thus m i l k s u g a r , 80 + IOC + l 6 H , c o u l d be c o n v e r t e d t o mucic a c i d , 80 + 6C + 10H, by t h e r e m o v a l o f f o u r c a r bon atoms and s i x hydrogen a t o m s . Such s p e c u l a t i o n s may on o c c a s i o n have p r o v e d u s e f u l t o B e r z e l i u s , but when he came t o c o n s i d e r s p e c i f i c problems i n w h i c h s t r u c t u r a l n o t i o n s might have been u s e f u l , he c l e a r l y r e g a r d s the m u t u a l s a t i s f a c t i o n o f e l e c t r o c h e m i c a l a f f i n i t i e s as the d e t e r m i n i n g f a c t o r , t o w h i c h n o t i o n s i n v o l v i n g arrangement can l e n d a t b e s t c i r c u m s t a n t i a l support. F o r example, i n 1839 B e r z e l i u s w r o t e t o Pelouze t h a t he had asked t h e Academy i n S t o c k h o l m : 'What i s the g r e a t e s t number o f oxygen atoms w h i c h can be combined i n a s i n g l e o x i d e w i t h a s i n g l e o r compound r a d i c a l ? ' Yet he makes no m e n t i o n o f g e o m e t r i c d i s t r i b u t i o n , and i t seems l i k e l y t h a t he was t h i n k i n g m e r e l y i n terms o f c o m b i n i n g p r o p o r t i o n s and t h e m u t u a l s a t i s f a c t i o n of e l e c t r o c h e m i c a l a f f i n i t i e s . A more s t r i k i n g i n s t a n c e i s p r o v i d e d by M i t s c h e r l i c h ' s d i s c o v e r y o f t h e p r i n c i p l e of isomorphism i n B e r z e l i u s ' s l a b o r a t o r y i n 1819 *. B e r z e l i u s i n i t i a l l y h e s i t a t e d t o a c c e p t i s o morphism, but checked M i t s c h e r l i c h ' s r e s u l t s a t f i r s t hand, and was c o n v i n c e d . He w r o t e t o t h e c r y s t a l l o g r a p h e r Hauy, whose t h e o r y was c o n t r a d i c t e d by


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C a l c i t e c r y s t a l s and models ( c a . 1805) i l l u s t r a t i n g Haûy's t h e o r y . Photograph by G. L'E. T u r n e r .


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M i t s c h e r l i c h ' s f i n d i n g s , and who t h e r e f o r e remained unc o n v i n c e d , t h a t A new f i e l d i s o p e n i n g h e r e , we a r e becoming aware o f a whole new c l a s s o f s u b s t a n c e s which, while conserving absolute r e g u l a r i t y o f t h e i r f o r m , can v a r y i n t h e i r elements between c e r t a i n l i m i t s , w i t h o u t t h i s v a r i a t i o n b e i n g d e t e r m i n e d by c h e m i c a l a f f i n i t y , and w i t h o u t i t c o n f o r m i n g t o t h e law o f c h e m i c a l p r o p o r t i o n s ; i t d e r i v e s from the s i m p l e c i r c u m s t a n c e s t h a t t h e s e d i f f e r e n t s u b s t a n c e s oan form i n t e g r a n t p a r t s o f t h e same c r y s t a l l i n e f o r m . Isomorphism c l e a r l y had much t o c o n t r i b u t e t o c h e m i c a l and c r y s t a l l o g r a p h i c t h e o r y , and B e r z e l i u s d i d a l l o w i t t o modify h i s m i n e r a l o g i c a l c l a s s i f i c a t i o n , but i n a c h a r a c t e r i s t i c and s i g n i f i c a n t way. I n h i s Attempt to 6 6

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1 6 7

e s t a b l i s h a pure s c i e n t i f i c system a p p l i c a t i o n of the Electrochemical

of mineralogy by the theory and the

Chemical P r o p o r t i o n s o f 1814, he had a r r a n g e d m i n e r a l s a c c o r d i n g t o the e l e c t r i c a l c h a r a c t e r o f t h e i r p o s i t i v e constituents. Once M i t s c h e r l i c h h a d demonstrated i s o morphous replacement among t h e s e c o n s t i t u e n t s , B e r z e l i u s ' s c l a s s i f i c a t i o n became i n v a l i d , and i n 1824 he p r o p o s e d a new system — t h i s time based on the e l e c t r i c a l c h a r a c t e r i s t i c s o f t h e negative constituents . Ideas i n v o l v i n g s t r u c t u r e might have something t o t e l l c h e m i s t s , might even o c c a s i o n a l l y p o i n t out e r r o r s — but a l t h o u g h s t r u c t u r e and r e l a t i v i t y were i n t i m a t e l y c o n n e c t e d , B e r z e l i u s ' s c l a s s i f i c a t i o n was always based upon the n a t u r e o f atoms, whose f o r c e s i n t e r a c t e d t o produce t h e secondary phenomena o f arrangement. Theories which g r e a t l y d e p r e c i a t e d e i t h e r n o t i o n s o f arrangement o r n o t i o n s o f atomic n a t u r e s c o u l d not s u p p o r t s t e r e o c h e m i c a l i d e a s , w h i c h needed b o t h atoms and arrangement. So t h e t h e o r y o f t y p e s and e l e c t r o c h e m i c a l d u a l i s m had e i t h e r t o merge o r t o g i v e way a l t o g e t h e r b e f o r e t h e concept o f s t e r e o c h e m i s t r y c o u l d become p o s s i b l e , l e t a l o n e c r e d i b l e f o r t h e community o f c h e m i s t s . The two t h e o r i e s were i n f a c t m o d i f i e d and merged, g r a d u a l l y , and as e a r l y as 1853 L i e b i g was able t o w r i t e t o Gerhardt, ' I t i s very strange that t h e two t h e o r i e s , f o r m e r l y q u i t e opposed, a r e now comb i n e d i n one w h i c h e x p l a i n s a l l the phenomena i n t h e two s e n s e s ' . R a d i c a l s had ceased t o f u n c t i o n f o r c h e m i s t s as t h e a l l but immutable o r g a n i c analogues o f i n o r g a n i c e l e m e n t s , and might even s u f f e r s u b s t i t u t i o n t o o c c u r . The t h e o r y o f t y p e s , on the o t h e r hand, was m o d i f i e d so as t o a l l o w f o r t h e r o l e o f i n d i v i d u a l atomic n a t u r e s i n d e t e r m i n i n g m o l e c u l a r p r o p e r t i e s . M e r g i n g , however, was n o t enough. The s t r u c t u r a l a s p e c t s o f t h e t h e o r y o f t y p e s h a d i n i t i a l l y been d e r i v e d from c r y s t a l l o g r a p h i c a n a l o g i e s , based upon t h e 6 8

6 9

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concept o f a f u n d a m e n t a l n u c l e u s . Other and e a r l i e r c r y s t a l l o g r a p h i c a n a l o g i e s had used t h e concept o f c l o s e p a c k i n g t o d e r i v e s t r u c t u r e . N e i t h e r approach s u f f i c e d f o r t h e development o f s t e r e o c h e m i s t r y , w h i c h emerged a f t e r t h e development o f v a l e n c e t h e o r y , and d i r e c t i o n v a l e n c e t h e o r y a t t h a t . The t h e o r y o f t y p e s was u s e f u l h e r e : but the t h e o r y o f t y p e s p r o v i d e d above a l l a c l a s s i f i c a t i o n w h i c h was a r t i f i c a l . What was needed was a d e s c r i p t i o n w h i c h was r e a l i s t i c . That i s why v a n ' t H o f f i n 1874 w r o t e : I t appears more and more t h a t t h e p r e s e n t c o n s t i t u t i o n a l f o r m u l a e a r e i n c a p a b l e o f e x p l a i n i n g c e r t a i n oases o f i s o m e r i s m ; t h e r e a s o n f o r t h i s i s perhaps t h e f a c t t h a t we need a more d e f i n i t e s t a t e m e n t about the a c t u a l p o s i t i o n o f the atoms' *. As h i s s t a t e m e n t i m p l i e s , f o r m u l a e i l l u s t r a t i n g r e l a t i o n s o f v a l e n c y were a l r e a d y w i d e l y assumed t o convey some i n f o r m a t i o n about t h e r e l a t i v e d i s p o s i t i o n o f atoms. The t e t r a v a l e n c y o f c a r b o n had been v a r i o u s l y e s t a b l i s h e d , by O d l i n g , Kekulé, K o l b e , F r a n k l a n d , and C o u p e r . P i c t o r i a l r e p r e s e n t a t i o n s o f compounds o f t e t r a v a l e n t c a r b o n r a i s e d i m p o r t a n t quest i o n s , f o r they appeared t o r e p r e s e n t t h e p h y s i c a l d i s p o s i t i o n o f atoms. Crum B r o w n was among t h e f i r s t t o 7 2

7 3


f

71

7 5

7 7

(A.S.

0

OH

0

OH Couper)

7 6

Kekulé

Crum Brown

pursue such q u e s t i o n s c r i t i c a l l y . Kekulé came by t h e e a r l y 1860s t o a p o s i t i o n where he b e l i e v e d t h a t ' i t must now i n d e e d be h e l d as a t a s t e o f n a t u r a l s c i e n c e t o a s c e r t a i n t h e c o n s t i t u t i o n o f m a t t e r , and t h e r e f o r e , i f we c a n , t h e p o s i t i o n o f t h e a t o m s ' , and a l t h o u g h he r e f u s e d t o r e a c h premature c o n c l u s i o n s about s t e r e o c h e m i s t r y , h i s v i e w s came t o be seen as s u p p o r t i v e o f the s t e r e o c h e m i c a l e n t e r p r i s e . The f o r m a l i n t r o d u c t i o n o f t h e n o t i o n o f s t r u c t u r e came, however, not from K e k u l e , but f r o m B u t l e r o v , who a s s e r t e d i n 1861: ' S t a r t i n g from t h e a s s u m p t i o n t h a t each c h e m i c a l atom p o s s e s s e s o n l y a d e f i n i t e and l i m i t e d amount o f chemical f o r c e ( a f f i n i t y ) w i t h which i t takes part i n f o r m i n g a compound, I might c a l l t h i s c h e m i c a l a r r a n g e ment, o r t h e t y p e and manner o f t h e m u t u a l b i n d i n g o f 7 0


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t h e atoms i n a compound s u b s t a n c e , by t h e name o f "chemical s t r u c t u r e " . The achievement o f v a n t H o f f and o f Le B e l was l a r g e l y t h a t o f f u r n i s h i n g a concept u a l t o o l w h i c h w o u l d e n a b l e c h e m i c a l s t r u c t u r e t o be r e p r e s e n t e d s y s t e m a t i c a l l y and w i t h p r e c i s i o n . They r e f i n e d t h e t h e o r y o f s t r u c t u r e and d i d i n d e e d p r o v i d e a more d e f i n i t e s t a t e m e n t about t h e a c t u a l p o s i t i o n o f t h e atoms'. When B u t l e r o v f i v e y e a r s l a t e r d e f i n e d s t r u c t u r e as t h e arrangement o f c h e m i c a l bonds between atoms i n a p a r t i c l e , he was b e i n g b o t h comprehensive and r e t r o s p e c t i v e . 7 9

f

f


8 0

L i t e r a t u r e Cited 1. van't Hoff, Jacobus Henricus, ' V o o r s t e l tot u i t breiding der tegenwoordig i n de scheikunde gebruikte structuur-formules i n de ruimte, benevens een daarmee samehangende vermogen en chemische c o n s t i t u t i e van organische verbindingen', J. Greven, Utrecht, 1874. Translated as 'Sur les formules de structure dans l ' e s p a c e ' , Archives Neerlandaises des Sciences Exactes et Naturelles (1874) 9, 445-454. 2. Le B e l , Joseph A c h i l l e , 'Sur les r e l a t i o n s qui existent entre les formules atomiques des corps organiques, et le pouvoir r o t a t o i r e de leurs d i s s o l u t i o n s ' , B u l l e t i n de la Societe Chimique de France, [2], (1874) 22, 337-347. 3. e.g. Hofmann, A . W . , 'On the Combining Power of Atoms', Chemical News (1865) 12., 166, 175, 187; Odling, W., 'Outlines of Chemistry', London, 1870, preface. 4. K e k u l é , Α . , 'Lehrbuch der organischen Chemie', v o l . 1, pp. 157-158, Erlangen, 1861. 5. cf. F i s h e r , N.W., 'Kekulé and organic c l a s s i f i c a t i o n , Ambix (1974) 21, 29-52. 6. See Brock, W.H. (ed.) 'The Atomic Debates. Brodie and the Rejection of the Atomic Theory', Leicester University Press, 1967. 7. 'Lehrbuch', op. c i t . Kekulé introduced his formulae i n v o l . 1, p. l65n. See R u s s e l l , C . A . , 'The History of V a l e n c y ' , p. 98, L e i c e s t e r University Press, 1971. 8. See Knight, D . M . , 'Atoms and Elements', Hutchinson, London, 1967, passim. 9. S t a l l o , J.B., 'The Concepts and Theories of Modern P h y s i c s ' , p. 126, reprinted by the Belknap Press of Harvard University Press, Cambridge, Massachusetts, I960. 10. Dalton, J., 'A New System of Chemical Philosophy', p. 216, Manchester, 1808. 11. I b i d , pp. 217, 219.



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12. Wollaston, W., 'On Super-acid and Sub-acid S a l t s ' , Philosophical Transactions of the Royal Society of London (1808) 98, 96-102. 13. e.g. Davy, Η . , 'Collected Works', ed. Davy, J., v o l . 5, pp. 435-6, London, 1839. 14. Laurent, Auguste, 'Methode de chymie', P a r i s , 1854; trans. Odling, W., 'Chemical Method', London, 1855. 15. 'Chemical Method', p. xv. 16. cf. F i s h e r , N.W., 'Organic c l a s s i f i c a t i o n before K e k u l é ' , Ambix (1973) 20, 108. 17. Laurent, op. cit., pp. x v - x v i . 18. Comte, Α . , 'Cours de philosophie p o s i t i v e ' , v o l . 4-5, P a r i s , 1830. 19. Brock, op. cit., pp. 145-152, has pertinent com ments i n his appendix on Comte, Williamson, and Brodie. 20. Comte, op. cit., v o l . 3, pp. 104ff. Levere, T.H., ' A f f i n i t y and Matter. Elements of Chemical Philosophy 1800-1865', p. 178, The Clarendon Press, Oxford, 1971. 21. Op. cit., v o l . 3, p. 109. 22. Laurent, Α . , doctoral t h e s i s , 1836, MS i n the archives of the Académie des Sciences, P a r i s . Part of t h i s was published i n 'Annales de Chimie et Physique' (1836) 61, 125; the rest i s published i n B u l l e t i n de Société Chimique de France (Documentation) (1954) 31, ed. J.Jacques. 23. Gerhardt and Chancel, Compte rendu (mensuel) des travaux chimiques (1851) 7, 65-84. 24. Laurent, op. c i t . 25. Laurent, Compte rendu hebdomadaire des s é a n c e s de l ' A c a d é m i e des Sciences (1842) 15, 350-352. 26. I b i d , (1840) 11, 876. 27. This example i s discussed by Kapoor, S . C . , 'The Origins of Laurent's Organic C l a s s i f i c a t i o n ' , I s i s (1969) 60, 513, and F i s h e r , N.W., op. cit., 119. 28. I b i d , 119. 29. Grimaux, Ε., and Gerhardt, Ch. j u n . , 'Charles Gerhardt, sa v i e , son oeuvre, sa correspondence', p. 481, P a r i s , 1900. 30. I b i d , pp. 483-487. 31. Levere, op. cit., passim. 32. Op. cit., v o l . 3, P. 50. 33. 'Chemical Method', pp. 321-322. 34. Levere, op. cit., pp. 167-169. 35. Ibid. 36. Daubeny, C . , Report of the B r i t i s h Association for the Advancement of Science (1856) 136ff. 37. Wurtz, A d . , 'Dictionnaire de Chimie', v o l . 1, p. 77, P a r i s , 1869. 38. Beudant, F.S., "An Inquiry into the Connection



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between C r y s t a l l i n e Form and Chemical Composition', Journal S c i . A r t s . (1819) 6, 117. 39. Dana, J.D., 'System of Mineralogy', p. 76, 2 e d . , 1844. 40. Comte rendu...de l ' A c a d é m i e des Sciences (1843) 17, 311-312. 41. Gerhardt, C., 'Introduction à l'étude de l a chimie par le s y s t è m e u n i t a i r e ' , p. 55, P a r i s , 1848. 42. Quesneville's Revue Scientifique et I n d u s t r i e l l e (1840) 1, 35-38. 43. 'Lehrbuch', v o l . 1, pp. 157-158, c i t e d i n t r a n s l a t i o n by R u s s e l l , op. cit., pp. 143-144. 44. Op. cit., pp. 35-36. 45. Op. cit., pp. 157-158. 46. 'Chemical Method', pp. 65-66, 68 dicusses synoptic formulae, which are analogous for analogous bodies. The formula for chloracetic acid i s from 'Annales de chimie' (1836) 63, 377 (388). 47. Op. cit., p. 157. 48. 'Alembic Club Reprint' no. 16, from Williamson's paper of 1850. 49. See, e.g., Davy, Η . , Philosophical Transactions of the Royal Society of London (1814) 104, 557-570. 50. Jahres-Bericht,(l840) 20, ii, 13. 51. S c i e n t i f i c Memoirs (1846) 4, 240-252. 52. ' J a c . Berzelius Bref, éd., Soderbaum, v o l . 7, p. 200, Uppsala, 1912-1932. 53. Rapport Annuel sur les P r o g r è s de l a Chimie (1845) 5, 21-22. 54. Brodie, B.C., 'On the Atomic Weight of G r a p h i t e ' , Philosophical Transactions of the Royal Society of London (1859) 149, 249-25955. cf. Daubeny, C., 'Introduction to the Atomic Theory', pp. 182, 431, 2 e d . , Oxford, 1850. 56. Report of the B r i t i s h Association for the Advancement of Science (1864) 24. 57. 'Charles Gerhardt, sa vie...', p. 38. 58. I b i d , pp. 42-43. 59. Kolbe, H., J. prakt. Chem. [2] (1877) 15, 47360. 'De Verbeeldingskracht i n de Wetenschap', pp. 4-24, trans. Benfey, O . T . , Journal of Chemical Educat i o n (I960) 37., 467ff. 61. Compte rendu...de l ' A c a d é m i e des Sciences (1840) 10, 171, 176, 178. 62. Annals of Philosophy (1815) 5, 274. 63. Compte rendu...de l ' A c a d é m i e des Sciences (1839) 8, 352-357. 64. M i t s c h e r l i c h , Ε., Abhandlungen der K ö n i g l i c h e n Akademie der Wissenschaften zu B e r l i n (1819) 427; trans. i n Ann. Chem. Phys. (1820) l 4 , 172.



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65. 'Jac. Berzelius B r e f ' , v o l . 7, p. 124. 66. I b i d , v o l . 7, 18 October 1821. 67. I b i d , v o l . 7, P- 162. 68. Annals of Philosophy, N.S. (1826) 11, 381, 422. 69. Quoted by Partington, J.R., 'A History of Chemis try', v o l . 4, p. 460, MacMillan, London, 1964. 70. Levere, op. cit., pp. 185-19371. See Laurent's thesis (22), and Mauskopf, H., Annals of Science (1969) 25, 229-242. 72. R u s s e l l , op. cit., pp. 159-16773. cf. F i s h e r , Ambix (1973) 20, 108. 74. Trans. Benfey, O . T . , i n 'Classics i n the Theory of Chemical Combination', p. 151, Dover, New York, 196375. See R u s s e l l , op. cit., pp. 119ff. 76. See Benfey, op. cit., p. 148; R u s s e l l , op. cit., 98, 102; Larder, D . , Ambix (1967) 14, 112-132. 77. Transactions of the Royal Society of Edinburgh (1864) 23, 707. 78. Cited by R u s s e l l , op. cit., p. 144. 79. I b i d , p. 149. 80. 'The Present Day Significance of the Theory of Chemical S t r u c t u r e ' , Journal of the Russian Physico Chemical Society (1879) 11, 289-311; reprinted i n G.V. Bykov, e d . , 'Centenary of the Theory of Chemical S t r u c t u r e ' , pp. 111-129, Moscow, 1961.


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