The Evolution of Chemistry. 131 



Dalton shows us that Aristotle is wrong, so Epicurus must 

 be right. The crucial^ test lay in the law of definite and 

 multiple proportions which the ancients knew nothing about. 

 Every discovery since made has confirmed the idea. No 

 one has been able to advance an alternative hypothesis that 

 could face the facts. Occasionally we hear of some chemi- 

 cal Eev. Mr. Jasper who tells his class that although he 

 teaches the atomic theory, he does not believe it. Some far- 

 reaching teleological speculation that he is ashamed to pub- 

 lish dominates his thoughts. Some spook of the imagina- 

 tion answering to no facts of experience, but maintaining 

 the continuity of his heritage of superstition, prompts the 

 utterance. For modern chemistry the atomic theory is the 

 only satisfactory one. Of course the atoms are not believed 

 to be the " uncuttable " things of Democritus. They are 

 minute, organized bodies of some kind, having as real an 

 existence in the world of fact as ourselves. In the working 

 out of Dalton's idea Berzelius took an active part, but, as 

 both confined themselves to gravimetric estimations, con- 

 firmation from a new standpoint was reserved for Gay-Lus- 

 sac. His volumetric study of gases revealed the fact that 

 they invariably unite in definite and multiple volumes. A 

 cubic foot of chlorine unites with a cubic foot of hydrogen 

 only. A cubic foot of oxygen unites with two cubic feet of 

 hydrogen. Sir Humphry Davy assisted him in this inves- 

 tigation. 



In 1811 Avogadro declared that equal volumes of any two 

 or more gases under the same temperature and pressure con- 

 tain the same number of molecules. All gases were found 

 by him to contract or expand in the same degree for the 

 same subtractions or additions of pressure or temperature. 

 No other hypothesis than this one of Avogadro's has been 

 advanced to explain why gases behave as they do. No other 

 is needed, as this matches the facts accurately. That gases 

 are discrete is proved by the fact that a volume of a heavy 

 and light gas when mixed does not make two volumes. The 

 one occupies the interspaces of the other, which it could not 

 do if they were continuous. This discovery of Avogadro's 

 came before chemists were prepared to receive it. The dis- 

 tinction which he made between an atom and a molecule 

 sorely puzzled his contemporaries. They did not see that 

 his molecule was a family of atoms. It is a moving, com- 

 pound unit capable of chemical decomposition into several 

 atoms. His law was only true of molecules and not of 



