Manchester Memoirs, Vol. xlvii. (1903), No. 11. 21 



just as Avogadro's law is applied to the determination ot 

 molecular weights among gases, so van't HofiPs rules 

 enable us to measure the molecular weights of substances 

 in solution. The atom, the molecule, and the molecular 

 weight enter into all of these new generalisations. In 

 short, if we take the atomic theory out of chemistry, we 

 shall have little left but a dust-heap of unrelated facts. 



I have now indicated, briefly and in outline only, the 

 influence of the atomic theory upon the development of 

 chemical thought. Details have been purposely omitted ; 

 the salient facts are enough for my purpose, and they 

 make, at least for chemists, an exceedingly strong case. 

 The convergence of the testimony is remarkable, and 

 when we add to the chemical evidence that which is 

 offered by physics, the theory becomes overwhelmingly 

 strong. This side of the question I cannot attempt to 

 discuss, but I may in passing just refer to Professor 

 Riicker's presidential address before the British Associa- 

 tion in 1 901, which covers the ground admirably. The 

 atomic theory has had no better vindication. 



And \-et, from time to time, we are told that the 

 theory has outlived its usefulness, and that it is now a 

 hindrance rather than a help to science. Some of the 

 objectors are quite dogmatic in their utterances; some 

 only seek to evade the theory, without going to the 

 extreme of an absolute denial ; and still others, more timid, 

 assume an apologetic tone, as if the atom were something 

 like a poor relation, to be recognised and tolerated, but 

 not to be encouraged too far. Now caution is a good 

 thing, if it is not allowed to degenerate into indecision ; 

 when that happens, mental obscurity is the result. In 

 science we must have intellectual resting-places ; some- 

 thing to .serve as a foundation for our thinking ; something 

 concrete and tangible in form. No theory is immune 



