44 PRINCIPLES OF CHEMISTRY. 



law of Avogadro, which has been stated heretofore, and says 

 that all gases under equal conditions contain the same number 

 of molecules. 



The weighing of equal volumes of gases is consequently 

 identical with the weighing of equal numbers of molecules. 

 The molecular weight of a substance can therefore be found by 

 weighing this substance in the gaseous state and comparing with 

 it the weight of an equal volume of another gas, the molecular 

 weight of which is known. The gas usually adopted for this 

 comparison is hydrogen. 



If, for instance, we weigh equal volumes of hydrogen, chlorine, 

 oxygen, hydrochloric acid, and steam, we find weights in the 

 proportion of 2, 71, 32, 36.5 and 18. These numbers at the 

 same time express the molecular weights of these substances, 

 but the numbers also show that atomic and molecular weights 

 of elements are not identical, but that the latter weight is twice 

 that of the atomic weight, or that the molecules of elements con- 

 sist of two atoms. 1 



Law of equivalents. Quantivalence. When one element re- 

 places another element in a compound, the quantities of the two 

 elements are said to be equivalent to each other, and according 

 to the law of equivalents the replacement of elements one by 

 another always takes place in definite proportions. Formerly 

 it was believed that all atoms were equivalent amongst each 

 other, and, accordingly, atomic \veights were frequentl} 7 desig- 

 nated as equivalent weights. 



This view, however, is not correct, as it is found that fre- 

 quently one atom of one element displaces two or more atoms 

 of another element. This fact, as well as other considerations, 

 has led to the assumption of the quantivalence of atoms. 

 This property will be best understood by selecting a few com- 

 pounds of different elements with hydrogen, for consideration. 



I. II. III. iv. 



HCl H,0 H 3 N H 4 C 



HBr H 2 S H 3 As H 4 Si 



HI H 2 Se H 3 P 



1 A few exceptions to this general rule will be mentioned at the proper places. 



