﻿Dr. A. Crum Brown on Chemical Constitution. 397 



of phosphorus, we find that one atom of oxygen (from the water) re- 

 places, and is replaced by two atoms of chlorine from the penta- 

 chloride; thus PCL-f-H 2 = PCl 3 + 2HC1 ; so that while the two 

 atoms of hydrogen were formerly united to one atom of oxygen 

 and formed with it one molecule, they are, after the change, each 

 united to a separate atom of chlorine and form with them two mo- 

 lecules. 



Oxygen, therefore, in this case (and, as far as we know, in all 

 cases) enters into two relations, while hydrogen, chlorine, silver, 

 and sodium only enter into one. In a similar way it has been 

 shown that the different elements have different "atomicities," or 

 enter into different numbers of relations. It is to this " polyatomi- 

 city," or multiple •-? -elatedness , that the complexity of compounds 

 is due ; for it is obvious that by the union together of several mul- 

 tiply related atoms a very complicated structure may be produced. 



In the case of a compound containing only two atoms, such as 

 chloride of sodium, there is clearly only one way in which it can 

 break into residues; but a complex substance, containing many 

 atoms, may, and generally does, break in different ways when acted 

 on by different substances ; and it is by the study of these ways of 

 decomposition of a substance, and by the study of the ways in 

 which, by means of double decomposition, it can be produced, that 

 we arrive at a knowledge of its structure — that is, of the mutual 

 relation of its atoms. 



But the multiple -relatedness of some atoms produces a further 

 complication, producing a kind of chemical action which, while still 

 a chemical exchange, cannot be called double decomposition. In 

 double decomposition we saw that each molecule breaks into residues 

 which change places with the residues of the other molecule, and 

 that this breaking into residues results from the rupture of one or 

 more relations between pairs of atoms. But where we have multi- 

 ply related atoms, it may happen that such a rupture takes place 

 without a separation of the residues, these being retained in combi- 

 nation by some other relation of their multiply related atoms. To 

 illustrate this we may compare the action of anhydrous potash, K 2 0, 

 and of anhydrous lime, CaO, on water. 



Using graphic formulas, we have in these two cases : — 



0— ^ ®— ©-r© 



and 



&~(b + ©-!-©-© = 



-i-®-© 



r 



Ca) 



Here the dotted lines indicate the relations ruptured ; and it will 

 be seen that, while in the first case the rupture produces a separa- 

 tion into two residues, in the second case it does not, what would 

 otherwise be residues remaining united on account of the double 

 relatedness of the calcium atom. 



