330 HISTORICAL INTRODUCTION TO CHEMISTRY CHAII 



and of temperature a remarkable conclusion that has no 

 parallel in the behaviour of liquids or solids. 



B. THE COMBINING VOLUMES OF GASES 



Cavendish (1781) measures the combining volumes of 

 hydrogen and oxygen. In his experiments on the com- 

 position of water Cavendish found that 423 measures of 

 hydrogen were just sufficient to remove the oxygen from 

 1000 measures of air. If we assume that 20-9% of the 

 atmosphere consists of oxygen, this would mean that 209 

 measures of oxygen combine with 423 measures of hydrogen 

 or 100 measures of oxygen with 202 measures of hydrogen. 

 He also found that a mixture of 19500 grain measures of 

 oxygen and 37000 of inflammable air, after a series of 

 explosions, left a residue of 2950 grain measures of gas, of 

 which about 1000 consisted of unburnt oxygen. If it be 

 assumed that the remaining 1950 grain measures consisted 

 of impurities, shared between hydrogen and oxygen in pro- 

 portion to their original volumes, we find that 19500 - 1000 

 -650=17850 measures of oxygen combined with 37000 

 [ - 1300 = 35700 measures of hydrogen; that is, one volume 

 of oxygen combined exactly with two volumes of hydrogen. 



Gay-Lussac and Humboldt (1805) measure the 

 combining volumes of hydrogen and oxygen. In the 

 year 1805, Gay-Lussac and Humboldt (Jour, de Physique^ 

 1805, 60, 129) attempted to determine the "Ratio of the 

 Constituents of the Atmosphere" by exploding air with 

 hydrogen in a Volta's eudiometer and measuring the 

 diminution of volume which resulted. In order to 

 interpret their observations they found it necessary to make 

 a fresh determination of the ratio by volumes in which 

 hydrogen and oxygen unite. The most trustworthy series 

 of experiments was one in which 100 parts .of oxygen (of 

 which 99*6 parts could be absorbed by potassium sulphide) 



