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CONSTITUTION OF CHEMICAL COMPOUNDS. 



IT is noticed that certain elements combine with each 

 other in only one proportion, forming thus but one kind of 

 compounds. If we take, for instance, hydrogen and chlo- 

 rine, and allow them to combine under the most varied 

 conditions, the result is always hydrochloric acid, and this 

 always contains 35.5 parts by weight of chlorine to 1 part 

 by weight of hydrogen. The same is true of a number of 

 other elements, as bromine, iodine, potassium, sodium, etc. 

 Further, we notice that in the case of other elements, as 

 oxygen and sulphur, nitrogen and phosphorus, carbon and 

 silicium, a greater variety presents itself in their combina- 

 tions, not only with each other, but with the elements of 

 the first class referred to. Oxygen combines with hydro- 

 gen in two proportions, forming water and hydrogen per- 

 oxide ; nitrogen combines with oxygen in five proportions, 

 forming nitrous oxide, hyponitric acid, nitrogen binoxide, 

 nitrons anhydride, and nitric anhydride. This distinction, 

 between elements that combine with each other only in one 

 proportion, and those which combine with each other and all 

 other elements in more than one proportion, is fundamental 

 and characteristic. The recognition of this distinction led to 

 the acceptation of the hypothesis of the valence of elements. 

 This hypothesis may be stated as follows : Every atom of 

 an element has an inherent power of holding in combina- 

 tion a certain number of other atoms of known combining 

 power. The simplest examples of this principle, we find 

 in the first class of elements mentioned above ; they com- 

 bine with each other in only one proportion, i. e., each 

 atom can retain in combination only one other atom of any 

 kind, and its combining power, as well as that of the atom 

 with which it is united, represents the unit of this power. 

 The atoms of such elements are said to possess one affinity ; 

 and the elements are called monovalent. 



In order to determine which elements are monovalent, 



