justification for writing thermochemical equations as for many years we have been writing them. I wish now that there were opportunity to speak of the methods whereby these nuclear reactions are detected, through observations on the fast-moving fragments which result from the reactions-observations which incidentally permit us to verify Einstein's principle, for what I have just been calling the "heat of reaction" is actually the kinetic energy of these fragments, and this can be ascertained from the data. These methods are strikingly different from the methods of ordinary chemistry, and very remarkable in themselves. The time, however, was not sufficientfor me to do justice both to the discoveries and to the methods, and perhaps it was the better policy to leave you wondering how these wonderful things were found out, rather than to have left you wondering what such clever methods are good for. Methods and data alike should figure in any general course on the subject; but no doubt you often have to impart significant items of knowledge without telling the class in full detail just how they were obtained, and you can do the like with these. It is strange and thrillmg to realize what bas bappened to our old doctrines of the elements and the atoms during these few past years. Not so very long ago we believed in immutable elements, and supposed without definite proof that they consisted of distinct and inalterable atoms. Now there are many admirable ways of detecting those atoms, but it turns out that they are not inalterable at all! Every year, nay, almost every month, brings the discovery of some new kind of elementary particle, and a t the.same time brings new evidence that all of these elementary particles are but different subdivisions of a single substance which can be caused to pass from one into another. I t would be excellent to communicate to our students this feeling of wonder and strangeness. Yet we ourselves feel it chiefly because our memories run back to the verge of the period, if not indeed far back into the period, when transmutation was impossible and the atoms of the various elements and of electricity and light seemed to be set apart sharply and permanently from one another. and now you see that the heat of reaction is both relaHow will you be able to take your classes back into tively and absolutely far more considerable than in the that period? Would it be possible to teach the beprevious case. The mass of the calories amounts to a ginners' course about inimutable elements and uncouple of hundredths of a gram--0.018 in the first changeable atoms from September to the end of March, equation, 0.024 in the second--and this is a very apand then on the 6rst of April* suddenly reveal how compreciable fraction of the other masses present in the pletely those doctrines have been superseded? If not, equation. To be precise, the masses on the left-hand I hope that you will find some other way of saving sides of these equations (m terms of one-sixteenth the transmutation for a little while from the destiny which mass of 0'3 are 8.022 and 8.028, respectively, while lies in wait for all discoveries-the fate of being quietly twice the mass of the helium atom is 8.004. The equaaccepted as a commonplace. tions would be unmistakably unbalanced if the calories were left out, but these are just of the right size to make the fifteenth of April, but accept with enthusiasm the them balance. Nuclear chemistry has taught us the amendment proposed by Chairman Baker at the meeting. bra teacher would talk to the legendary algebra pupil who wants to equate x horses with x cows. He had something very different in mind: he equated the weight of CaO to the sum of the weights of Ca and 0, and then he hung on the 100,000 calories as a kind of afterthought just to remind the student that so much heat is produced when the given quantities (grammolecules, I suppose) of Ca and 0 combine to form CaO. He did not really think that the last term belonged in the equation; he thought that the equation was correct without it; he put it there because he wanted it somewhere in sight, and could think of no better place to put it. And yet, that term does belong in the equation; the equation is not right without it; the thermochemists, when they wrote that term as if it belonged to the equation, were unconsciously correcting an error which they did not perceive. This I think is one of the most amusing things in the history of science. But if it was an error to suppose the equation complete without the 100,000 calories, why did not the chemists perceive this by observing that the mass of CaO is actually less than the sum of the masses of the Ca and the 0 of which it is formed? I can show the reason at once. According to Einstein's principle, the of one gram; mass of one erg of energy is less than that of the 100,000 calories is only about 5.10-9 g., and when you compare it with the 40 and the 16-g. which are the masses of the gram-molecules of Ca and 0 , respectively, you see that it is relatively so very small that we can scarcely hope ever to detect that the (cooled down) CaO is lighter than the Ca 0 by this amount. Such is the case with all the other reactions of ordinary chemistry. Far otherwise with nuclear reactions. Take the two involving lithium and hydrogen. I wrote them incompletely as equations (10) and (11); written completely, here they are:
+
yssaid
-
-
