D. R. Oldroyd School of History and Philosophy of Science University o f New South Wales Kensington 2033, Australia
Some Eighteenth Century Methods for the Chemical Analysis of Minerals
This paper examines briefly the development of mineral analysis in the eighteenth century, and descrihes some of the early inorganic analytical procedures, a number of which are still in use today. It also attempts to portray some of the difficulties which faced the early analysts, and shows how, by the end of the century, many of the theoretical difficulties a t least had been overcome. Even at the beginning of the century, it was pmsihle to do something towards a chemical analysis of mineral suhstances. The traditional methods of the assayers ( I ) could give reasonably accurate information about the proportions of metals which could be extracted from ores, though this was not necessarily the same as the proportions which were present in those ores. In addition, it was customary to submit the material being investigated to as high a temperature as possible in a furnace, without embarking on a full assay. These processes of "analysis by fire" were very uncertain, however, and numerous disputes arose as to whether the substances obtained when a material was submitted to the action of heat were actually "in" the material prior to the heating, or were produced in some way by the heating. Thus at this period most systems of classification were based upon external characteristics. By contrast, by the end of the eighteenth century, chemical methods of analysis were widely used and provided the basis for many systems of mineral classification. These advances became possible through a widening empirical knowledge of precipitation reactions, and of ways of bringing substances into solution. Also, most importantly, the recognition of the chemical individuality of the various "earths," serving as the last terms of chemical analysis, enabled chemists to develop a viable scheme of mineral analysis. Where did knowledge of these individual "earths" come from? Knowledge of calcareous earth as a individual substance dates hack for centuries, and we cannot tell who was the first to recognize its individuality. The first recognition of magnesia as an earth distinct from the calcareous kind appears to he due to the German medical chemist, Friedrich Hoffmann (1660-1742). Hoffmann showed that the white precipitate obtained when he added "salt of tartar" to the mother-liquor from evaporated brine, dissolved in "vitriolic acid" to give a bitter tasting solution (Epsom salt), whereas quicklime gave an almost tasteless, sparingly soluble salt (gypsum) when similarly treated with acid. He viewed the Epsom salt as a combination of "sulphurous acid" and a "mild and alkaline calcareous earth" (21. Evidently, Hoffmann realized that there was something which made this earth different from ordinary calcareous earth, though the significance of the observation was not perhaps realized a t this stage. According to the Swedish chemist, T. 0. Bergman (1735-84) (3), "terra ponderosa" was first recognized as an individual substance by his countrymen, J. G. Gahn (1745-1818) and C. W. Scheele (1742-861, whereas the German chemist, A. S. Marggraf (1709-82), had previously considered it to he a calcareous earth. The individuality of the heavy earth was subsequently confirmed by Bergman's own experiments. As might be expected, the main
Table 1. Bergman's Analytical Method
distinction was that the "terra ponderosa" gave a sparingly soluble "ponderous spar" with "vitriolic acid," whereas the corresponding gypsum was appreciably soluble. It was Marggraf who first recognized "argillaceous earth" as an individual entity. He informs us (4) that in his day alum was generally thought to he a salt composed of an " e a r t h and "vitriolic acid." His predecessors had regarded the "earth" as a calcareous substance, hut Marggraf detected significant differences. Thus a solution of "earth of alum" in "nitrous acid" gave no precipitate on treatment with "yitriolic acid," whereas a corresponding solution of chalk did give such a precipitate. Thus he recognized that he was dealing with a new kind of "earth.', Marggraf also made numerous other observations of fundamental importance, investigating many of the commoner inorganic precipitation reactions and the solvent actions of acids and alkalis upon his precipitates. In this work, he laid the foundations for what later became the tables of qualitative analysis which we studied in our youth. The first recognition of silica as an individual "earth" seems to have been due t o the German chemist, J. H. Pott (1692-1777) (5). He recognized four "primitive earths": calcareous, argillaceous, gypsous, and "vitrifiable," the last corresponding to our silica. His "vitrifiable earth" was largely characterized by its behavior on heating, though he noted its ready fusion on heating with alVolume 50, Number 5. May 1973 / 337
kali, and its resistance to attack by acids. By the middle years of the eighteenth century, then, a mass of qualitative data had been assembled which could be put to good use in the formulation of a general scheme of mineral analvsis. and in the work of ~ e r i m a nwe find the first published exposition of such ascheme. a he secret, of course, was to bring the whole of the mineral heing investigated into solution, which could he achieved by fusion with alkali. This reaction was not new. It had been known for centuries from the work of the glass manufacturers. Nevertheless, it was Bergman who first made use of the reaction as part of a general analytical procedure. His method is described in Volume 2 of the English translation of his "Physical and Chemical Essays," under the head: "Method by which the proximate Principles of Gems are most easily determined." F r p this, his general analytical scheme may be extracted and exhibited as in Table 1f6). Sadly, Bergman's published results were not a t all accurate, the errors, in part, being due to the fact that he had not taken some of the less common "earths" into consideration in his analyses. Also, some of the analytical results of his pupils were included in his own works. An additional source of error seems to have been the fact that his materials became contaminated with silica as a result of the grinding of his gems in his mortars. Nevertheless, Bergman's authority as an analytical chemist was so considerable that in many cases his work remained uncorrected for several years. However, the contribution offered to mineralogy by his procedures of "analysis in the humid way" was of fundamental significance, and the actual results obtained were of lesser importance. It may be noted in passing that this analytical scheme was devised by one of the chief exponents of the phlogiston theory. The phlogistonist could quote his analyses in terms of "earths," thinking of them as simple substances, 338 / Journal of Chemical Education
Table 2.
Vauquelin'o Analytical Method
Material ilinely powdered and uelphedl oddpotnrh, lhral, cnnl. drroiirin r o t r r
I
No,*:a,, ,pripita,nr were warhrd
hclen ik"ilii3".
Table 3. Klaproth's Analytical Method
since they appeared as the last terms of analysis. This procedure was subsequently taken over by the exponents of the new chemistry, and retained even after the earths were recognized a s compound suhstances. After Bergman took the first vital step. others soon followed his lead, notably Richard Kirwau (1733-1812) in Ireland, and Nicolas Vauquelin (1763-1829) in France. Both of these men used " h u m i d techniques. Some idea of the very rapid progress in analytical procedure may he gauged by comparing the general method of Vauquelin, given in Tahle 2 (71, with that of Bergman. Vauquelin's work was undoubtedly impressive, though his methods must have been exceedingly tedious, demanding very considerable skill on the part of the analyst. However, i t is in the work of the great German chemist, M. H. Klaproth (1743-1817), that one finds the culmination of eighteenth century methods of mineral analysis. When one considers the apparatus and materials which were available to him, Klaproth's skill and accuracy appear almost uncanny. Unlike Bergman, he quoted the actual weights of materials obtained in his analyses, so that errors were not concealed by arbitrarily making sure that the figures added up to 100%. He took scmpulous care to purify his reagents, was the first to dry or ignite to constant weight, and (unlike Bergman) allowed for the silica acquired from his mortars during the pulverization of his specimens. The precision and accuracy of his work aided the discovery of several new elements which had previously been overlooked. It is impossible to summarize Klaproth's methods hriefly, for, unlike Kirwan and Vauquelin, he did not publish
(Table 3. Continued)
At this stwe. Klaproth rathered tosahcr his lhree vrlloos d 'alumin%" (or slumioal I283 dllvre "ritrnlic acid-R pr. rcmllned undirsol"d, and unsrrealed as ri1ics: Solution
a general analytical scheme. Each of his analyses had its own individuality, each being subtly adapted to the nature of the substance under investigation. We take his analysis of the sapphire as our example (Table 3) (81, though it should he emphasized that this was one of the simplest investigations described in his work. Even here, the complexities of the problem are very apparent. A check on the operations had to be maintained a t all times with the aid of a balance, so that mistakes of the kind made by Bergman (when he contaminated his gems with
v.+
+ 1a.Land ireawd them w i t h
$6a.
+
RsridueIRer, i l e r bnm the "riler'". a h l l r ~ , ioirl,l,r,,don,l"l,an
I
Reddue I l i sr I
s,,!u,ic2n
Precil'itate
silica from his pestle and mortar) would he avoided. It can he seen, then, that once a technique for bringing minerals into solution had been established, and the geoeral notion of separating earthy suhstances by precipitation reactions had been put forward, it soon became possible to offer a genuine analysis. No longer, as a t the heginning of the eighteenth century, need one fear that one's "analysis by fire" would alter the "ingredients" of a substance during the course of the investigations. Minerals could be viewed as combinations of the several known Volume 50, Number 5, May 1973 / 339
earths and metals, and the relative proportions of each could be determined with considerable accuracy. With patience, and a skill which few can emulate today, it had mineral hecome possible to carry Out very analyses, thereby laying the basis for the chemical classification of the mineral kingdom. Literature Cited 11) See: Greonaway. F..Plor. IOlhlnt. Congr Hirr. Sci. 819(19621.
340 / Journal of Chemical Education
(21 Cked in: Partington. J. R., ..A Hiaton. of Chemia~y:. Mamillan, London, (19611. V o l 2, p. 696. 13) B.,,,,,, T.o.,l l ~ ~ i b r e r t e t i oonn mcctive ~ t t ~ ~.. ~ t i ~ 178.5. ~ ~ 180. . (41 ~ a r g p r a i . s.:~opuscu~eschymiquer.. ~. .(l z . ~ a r i s 117621, . p. lea. (51 Poft. d. H.. "Lilhogengnolie. . Paris. 1753, p. 7 andpossirn (61 Berman. T.O.;~Physics~ and Chemical Esaaya: 2, London, (17841. pp. 95.103. (71 vauqudin. ~ . . ~ ndne c. h i m . . 30.66117991. (81 Klspiolh. M. H.."Analytical Essays towards Pmmoting the Chemical Knowledge of Mineral Substances." Inndon, IROI, pp. 72 if. (First German ed.. Berlin, 17%)
.."
.
