102 PRINCIPLES OF CHEMISTRY. 



rine. Hence the molecules of hydrogen and chlorine must be divided to form 

 part of a product consisting of twice as many molecules, since each molecule of 

 hydrochloric acid contains some hydrogen and some chlorine. But if the mole- 

 cules of hydrogen and chlorine can be divided, they must consist of more than 

 one atom. Everything that is known about hydrochloric acid justifies the 

 assumption that its molecule contains one atom each of hydrogen and chlorine. 

 If this is true, it follows then that the molecule of hydrogen and of chlorine 

 contains two atoms. The same kind of argument in the case of the union of 

 hydrogen and oxygen to form water-vapor, leads to the conclusion that the 

 molecule of oxygen contains at least two atoms. In fact, from various experi- 

 ments and processes of reasoning, it has been established that the molecule of 

 nearly all the elements, in the gaseous state, consists of two atoms. 



Theory (Law) of equivalents. Valence, or Quantivalence. 

 When one element replaces 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 takes place always in definite proportions. Formerly 

 it was believed that the atoms of all elements were equivalent one 

 with another ; accordingly, atomic weights were frequently designated 

 as equivalent weights. 



This view, however, is not correct, as it ir> found that one atom of 

 one element frequently 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 

 understood best by selecting for consideration a few compounds of 

 different elements with hydrogen. 



i. ii. m. iv. 



HCl H 2 H 3 N H,C 



HBr H 2 S H 3 As H 4 Si 



HI H 2 Se H 3 P 



We see here that Cl, Br, and I combine with H in the proportion 

 of atom for atom ; O, S, Se combine with H in the proportion of 2 

 atoms of hydrogen for 1 atom of the other element ; N, As, P com- 

 bine with 3; C and Si with 4 atoms of hydrogen. 



Moreover, it has been found that the compounds mentioned in 

 column I. are the only ones which can be formed by the union of 

 the elements Cl, Br, and I with H. They invariably combine in this 

 proportion only. Other elements show a similar behavior. For 

 instance, the metal sodium combines with chlorine or bromine in one 

 proportion only, forming the compound Nad or NaBr. 



Looking at columns II., III., and IV., we see that the elements 

 mentioned there combine with 2, 3, and 4 atoms of hydrogen, 

 respectively. It is evident, therefore, that there must be some pecu- 



