TRANSACTIONS OF SECTION B. 'oGl 



Next, I would point out that in the atomic theory the notions of indivisiljility 

 and equality are inseparably involved. The indivisibility of atom and molecule 

 is not absolute or ultimate, and Dalton distinctly guarded himself against being 

 understood to claim for the atom more than chemical indivisibility, and chemists 

 of to-day assert no more than this. This indivisibility being conditioned by 

 the equality of niidecules, the importance of emphasising it rests only upon the 

 danger, when it is overlooked, of losing sight also of the chemical equality 

 through the gravimetric inequality receiving numerical expression, and thereby 

 conveying the notion of divisibility, though only gravimetrically. The idea 

 of indivisibility in connection with, the atom or molecule is intrinsically quite 

 subordinate to that uf equality ; for equality, being unity or oneness brought 

 into relation with itself, the conception of it carries with it and includes that of 

 indivisibility. .Vny rational hypothesis as to substances consisting of ultimate 

 particles will Include the notion of their being indivisible particles; and the 

 import of the hypothesis in cht>niical theory must lie, therefore, not in this 

 indivisibility, but in the nature of the equality of the jjarticles. By his atomic 

 theory l)alton asserted that where the substances are different this equality is 

 chemical instead of gravimetric. 



Molecules are equal in the sense that they are quantities of their sid)stances 

 which are interdependent and coordinate in any and every single chemical 

 change in which they take part together. It is a form of equality for which 

 no close parallel can be found ; but as to that it should be remembered that this 

 equality relates to the phenomena of the tranformations of substances into each 

 other, which, though they form so large a part of the phenomena of the universe, 

 are fundamentally distinct in nature from the rest of the behaviour of bodies 

 throughout which the substance remains what it was. In some agreement with 

 it there is that of mechanical pressures when these balance or neutralise each 

 other, and therefore are opposite and mutually destructive though equal. IJut 

 such pressures when exerted in the same direction are also equal in their effect 

 on any body in their path, whereas in chemical interactions the efi'ects of 

 molecules or equal quantities of two unlike substances are only equal in the 

 sense that each is that quantity which interacts with the same quantity of some 

 third substance, which itself proves to be also a chemically equal quantity to 

 them. For the products of the interaction in the one case are in part at least 

 not the same as those in the other, though all prove chemically equal in further 

 interactions. 



To give an example : the molecule of ammonia is equal to that of aldehyde 

 in that it combines with it and with it disappears, or ceases to exist as such. 

 For the same reason it is equal to the molecule of hydrocyanic acid, and molecules 

 of aldehyde and hydrocyanic acid equal to each other, because they, too, combine 

 and disappear as such in doing so. But the molecule of ammonia again equals 

 that of aldehyde in effecting transformation of hydrocyanic acid and its own 

 self into something else. And lastly, chemically equal or molecular are the 

 products of these combinations ; aldehyde ammonia, ammonium cyanide, and 

 aldehyde-cyanhydrine, not only among themselves, but also with the quantities of 

 ammonia, aldehyde, and hydrocyanic acid from which thej' come and into which 

 they return in other chemical changes. F>ut with all this quantitative equality 

 in transforming power, the substances produced are unlike and, each to each, 

 peculiar to one of the three acts of chemical combination ; and on this account 

 exception may be taken to the treatment of molecules as equal chemical quantities. 

 Yet the equality of molecules here asserted is but an extension of what is meant 

 by the equivalence of certain atoms and radicals, since the atom and the radical 

 are, nowadays, conceptions entirely dependent upon and derived from that of the 

 molecule (apart, of course, from the atomic hypothesis) ; and this universally 

 allowed equivalence admittedly does n^it extend to the identity of the products 

 of the replacing activity of the atoms and radicals. 



Quantitative equality and equivalency, it is true, have not the same meaning, 

 equivalence being used to denote qualified equality, equality in certain specitied 

 ways, of quantities not equal in all other ways and possibly in no other. Quantities 



1902. 



