THE BEGINNINGS OF LIFE. 51 



one another in that particular mode of collocation which 

 belongs to, and constitutes, the molecule in question. 

 The phenomena of allotropism, as we have previously 

 hinted, and various other considerations, tend to show 

 that even simple bodies such as phosphorus, sulphur, 

 and the metallic elements are made up of molecules 

 composed of similar atoms existing in a definite number 

 and grouping in each allotropic state or separate sub- 

 stance. An alteration of the number or grouping of 

 the atoms in the molecules, or of both, seems to be the 

 only way of accounting for the wholly different proper- 

 ties and crystalline form of one and the same sub- 

 stance, such as sulphur, under the influence of different 

 physical conditions. And thus vanishes the difference 

 between simple or elementary, and compound bodies. 

 They are all made up of molecules- only those of 

 the simple substances are aggregates of similar atoms, 

 whilst those of compound substances are aggregates of 

 dissimilar atoms. 



Different compound substances vary immensely in 

 their degree of complexity. Some, such as ordinary 

 acids or bases, are aggregates of simply complex mole- 

 cules; others are aggregates of doubly complex mole- 

 cules that is to say, two simply complex molecules 

 combine to form a doubly complex molecule, and, 

 when aggregated together, these include, amongst other 

 compounds, a very common class known as salts 1 . 



1 As a general rule, it may be said that decomposition follows the 

 reverse order. The larger molecules separate most easily; and the 



E 2 



