346 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



being taken as exactly 16, instead of 15*88. The atomic 

 weight of hydrogen then becomes roo8 instead of i and its 

 molecular weight 2-016 instead of 2 ; all other molecular 

 and atomic weights are increased in the same proportion. 



The choice of oxygen as a standard is justified by the fact 

 that, in practice, molecular and atomic weights are nearly 

 always measured by comparison with oxygen rather than 

 with hydrogen. The actual results of this comparison can 

 now be tabulated, whereas formerly the accuracy of the 

 tables was greatly impaired in reducing the numbers from 

 the oxygen to the hydrogen standard by means of a factor 

 deduced from the composition of water. 



Avogadro's hypothesis only approximately true. 

 Cannizzaro's method of deducing molecular weights has also 

 been modified by the discovery that Avogadro's hypothesis 

 is only approximately true. Like Gay-Lussac's " Law of 

 Volumes," it would probably be exact if applied to gases 

 under very low pressures. Several methods have been used 

 to eliminate the effects of unequal compressibility on the 

 relative densities of gases. The following table shows the 

 nature of the correction, and the large magnitude which it 

 attains in the case of easily-liquefied gases. 



