XX CONSTITUTION OF CHEMICAL COMPOUNDS. 



we have to subject the formulae and nature of their com- 

 pounds, as far as they are known, to the most careful 

 study. We thus find, in the first place, that hydrogen, 

 chlorine, bromine, iodine, etc., are monovalent. Having 

 once established this fact, knowing which elements are 

 monovalent, w r e have a basis upon which we can work to 

 determine the valence of other elements. Here again the 

 determination of the empirical formulae of the compounds, 

 of the elements to be investigated, with monovalent ele- 

 ments must be the first step in the inquiry. 



If we take oxygen, for example, we find that its simplest 

 compound with hydrogen is water, and by the aid of 

 familiar means we determine its formula to be H 2 0, i. e., it 

 consists of two atoms of hydrogen united with one atom 

 of oxygen. Hence we see that in this case, the atom of 

 oxygen exhibits a combining power, twice as great as that 

 of hydrogen, and, not finding any fact to conflict with this, 

 we say that oxygen is a bivalent element its free atom 

 possesses two free affinities. In a similar manner we find 

 that sulphur, selenium, tellurium, etc., are also bivalent. 



Proceeding further, nitrogen, phosphorus, arsenic, and 

 other elements are found to possess three times the com- 

 bining power of the monovalent elements ; their simplest 

 compounds with hydrogen are NH 3 , PH 3 , AsH 3 , etc. Ele- 

 ments of this class are called trivalent. 



Carbon, silicium, etc., are tetravalent, or the uncombined 

 atoms of these elements possess four free affinities. Their 

 simplest hydrogen compounds are CH*, SiH 4 , etc. . 



In this way all the elements have been classified into 

 groups, the individual members of which are said to be 

 monovalent, bivalent, trivalent, tetravalent, or pentavalent. 

 The elements are designated b}^ the names monads, dyads, 

 triads, tetrads, pentads, etc. This subdivision is dependent 

 merely upon the combining powers of the elements, and 

 tells us merely that the atoms of the elements of each 

 group can unite with, or hold in combination, a certain 

 number (indicated by the name) of monovalent atoms, such 

 as hydrogen, chlorine, etc. When we say an element is 

 monovalent, bivalent, trivalent, etc., we intend merely to say 

 that each one of its atoms possesses the power of combin- 

 ing with one, two, three, etc., monovalent atoms or atomic 

 units ; that each one of its atoms in the free state possesses 

 one, two, or three free affinities. 



Now compounds are formed by virtue of the mutual ac- 



