88 PROGRESS OF SCIENCE IN THE CENTURY. 



inaccuracies were discovered. A revision became 

 imperative, in which Liebig, Dumas, Stas, and others 

 took part. Different methods of determination were 

 discovered, one method was used to check another, 

 stimulus in the arduous task came at different periods 

 from the vision of supposed or real regularities con- 

 necting the different numbers (Prout and Meinecke 

 to Mendelejeff and Meyer), and gradually a well- 

 established, well-criticised system of atomic weights 

 was worked out. To Cannizzaro (1858) in particu- 

 lar credit is due for utilising the specific heat method 

 as a check on the others, and MendelejefFs periodic 

 law furnished, as will be seen, another valuable cor- 

 rective. 



It is a remarkable historical fact, however, that 

 owing to the relative unreliability of the methods 

 for determining the atomic weights, the conception 

 of the chemical atom fell for a time into general 

 disrepute. " At the end of the fourth decade of the 

 century, we find the atomic theory the most bril- 

 liant theoretical achievement of chemistry aban- 

 doned and discredited by the majority of chemists 

 as a generalisation of too hypothetical a character." 

 It was reserved for organic chemistry to re-vindicate 

 it, and for physical researches, especially on gases, 

 to place it on a yet firmer basis. 



Physical Enquiries and the Concept of the Mole- 

 cule. It is now necessary to allude to a path of 

 physical investigation which had a most important 

 influence on the atomic theory, especially through 

 Avogadro's Law and the kinetic theory of gases. 



In 1662, Boyle had stated, as Mariotte did some 

 years afterwards (1679), that the volume of a gas, 

 at the same temperature, is inversely as the pressure. 

 When the pressure increases, the volume diminishes 



