362 A SHORT HISTORY OF SCIENCE 



hydrogen. All the changes we can effect consist in the separation of 

 atoms bound together before and in the union of those previously 

 separated. 



The atomic theory while highly serviceable has always been 

 subjected to severe criticism. In 1840, for example, Dumas 

 declared that it did not deserve the confidence placed in it, and that 

 if he could he would banish the word " atom," convinced that 

 science should confine itself to what could be known by experience. 

 As late as 1852 Frankland says: 



I had not proceeded far, in the investigation of the organo-metallic 

 compounds before the facts brought to light began to impress upon me 

 the existence of a fixity in the maximum combining value or capacity 

 of saturation in the metallic elements which had not before been sus- 

 pected. ... It was evident that the atoms of zinc, tin, arsenic . . . 

 had only room, . . . for the attachment of a fixed and definite number 

 of the atoms of other elements. 



Independent researches have, in combination with the older chemi- 

 cal theories, introduced so much definiteness into this line of thought 

 that ' the Newtonian theory of gravitation is not surer to us now than 

 is the atomic or molecular theory in chemistry and physics so far, 

 at all events, as its assertion of heterogeneousness in the minute 

 structure of matter, apparently homogeneous to our senses, and to our 

 most delicate direct instrumental tests.' Kelvin, 1886. 



The three main criticisms of the atomic theory are : 

 (1) that it is based on inference, not on direct observation; 

 and is therefore only a provisional hypothesis ; (2) that it takes 

 no account of chemical forces "affinity"; (3) that it over- 

 emphasizes analysis. 



The idea of "atomicity" and "valency" . . . was not possible 

 without the clear notion of the "molecule" as distinct from the 

 "atom." This idea had lain dormant in the now celebrated but 

 long forgotten law of Avogadro, which was established in 1811 

 almost immediately after the appearance of Dalton's atomic theory. 



It had been known since . . . Boyle and Mariotte that equal 

 volumes of different gases under equal pressure change their volumes 

 equally if the pressure is varied equally, and it was also known . . . 



