PRESIDENTIAL ADDRESS MACKAY. 



throughout all chemical reactions. No difference how composite 

 it may be, if it maintains its unity throughout all chemical opera- 

 tions to which it can be subjected in the laboratory, then it is, so 

 far as the chemist is concerned, indivisible, and constitutes a 

 chemical atom. This is the conception of an atom that has long 

 prevailed in chemical circles. 



About ten years ago Sir Joseph Thomson's researches on the 

 nature of radiant matter revealed the existence of corpuscles a 

 thousand times smaller than the hypothetical hydrogen atom of 

 the atomic theory. This great discovery seems to have disturbed 

 the faith of weaker brethren, who imagined they saw in it the 

 approach of a cataclysm which would sweep away old landmarks 

 and leave few or none of our familiar chemical conceptions any 

 longer recognisable. But from what has now been said it will be 

 clear that while the discovery was one of extreme interest to 

 chemistry, it had no tendency to invalidate the atomic theory. 

 The tendency was rather in the opposite direction, since the dis- 

 covery furnished additional evidence of the existence of extremely 

 minute particles of matter. 



Just half a century after the publication of the atomic theory, 

 the progress of chemical knowledge and the corresponding evolu- 

 tion of chemical thought resulted in endowing the atom with a new 

 property, namely, a strictly limited capacity for combining with 

 other atoms, as measured by the number of atoms with which it 

 can combine. This is the property called valency. The facts 

 known were best explained by the assumption that a given atom 

 cannot become directly associated with or, figuratively speaking, 

 linked to, an indefinitely large number of other atoms. On the 

 contrary, the number is at most small, the atoms of each element 

 having a certain maximum capacity of combining. The capacity 

 of the atoms of some elements, hydrogen for example, is exhausted 

 when it has combined with one other atom. An atom of oxygen, 

 jn the other hand, can combine with two but with no more than 

 two such atoms as hydrogen. Or again, the limit of combination 

 for an atom of carbon is four atoms of hydrogen or two atoms of 

 oxygen. Hydrogen atoms accordingly are said to have a valence 

 of one, oxygen atoms of two, carbon atoms of four. The highest 



