LAWS OF CHEMICAL COMBINATION. 47 



It is needless to say that such representations are merely symbolical, 

 and express the view that atoms have a definite power to combine 

 with others. 



*When atoms combine with one another the bonds are said to be 

 satisfied, and it is graphically expressed thus : 



H H 



H Cl, H O H or O/ , H N H or N H 



X -H 



Whilst the valence of some elements is invariably the same under 

 all circumstances, other elements show a different valence (this means 

 a different combining power for other atoms) under different condi- 

 tions. For instance : Phosphorus combines both with 3 and 5 atomsV 

 of chlorine, forming the compounds PC1 3 and PC1 5 . As chlorine is/ 

 a univalent element, we have to assume that phosphorus has in one 

 case 3, in another case 5 points of attraction. Many similar instances 

 are known, and will be spoken of later. 



The only explanation which we can furnish in regard to the variability of 

 the valence of atoms is the assumption that sometimes one or more of the 

 bonds of an atom unite with other bonds of the same atom. If, for instance, 

 in the quinquivalent phosphorus atom two bonds unite with one another a 

 trivalent atom will remain. 



It is noticed invariably that the valence of atoms increases or diminishes by 

 two, which could not be otherwise, if the explanation given be correct. Thus 

 chlorine, the valence of which generally is I., may also have a valence equal 

 to III., V., or VII., while sulphur shows a valence either of II., IV., or VI. 

 Atoms whose valence is even, as in case of sulphur, are called artiads ; those 

 whose valence is expressed in uneven numbers, as chlorine and phosphorus, 

 are called perissads. 



While it is now being assumed that most of the elements possess more than 

 one valence, in consequence of the assumed power of bonds in the same atom 

 to saturate one another, in this book will be mentioned chiefly that valence 

 which the element seems to possess predominantly. 



The doctrine of the valence of atoms has modified our views of the 

 equivalence of atoms v We now say that all atoms of a like valence \ 

 are equivalent to each other.v' The atoms of each univalent element / 

 are equivalent to each other, and so of the atoms of any other valence, 

 but two atoms of a univalent element are equivalent to one atom of 

 a bivalent element, or two atoms of a bivalent element to one atom 

 of a quadrivalent element, etc. 



After having explained this valence of atoms, it now may be better 

 understood why the atoms in an element do not exist in a free or 



