THE NON-LIVING WORLD 145 



deemed to be the one explanation of the augmentation 

 and diminution of the bulk of bodies. Finally the 

 atoms of the chemical elements are supposed to be of 

 determinate specific gravities. These hypotheses are of 

 the greatest value for investigating, predicting, and pro- 

 ducing chemical changes of analysis and synthesis. 

 There is no need, however, to esteem them as more than 

 working hypotheses, still less to deem them a sufficient 

 and exhaustive explanation of the real nature of bodies. 

 Indeed there are various considerations which at present 

 absolutely forbid us so to regard them. 



We have seen how various substances, such as carbon, 

 sulphur, and phosphorus, may exist in two or more differ- 

 ent states, but there are certain others which can be made 

 to alternate between two conditions neither of which is 

 truly crystalline though one of them is allied to the latter 

 state. Thus the same chemical substance may sometimes 

 exist in a state which is termed crystalloid and at other 

 times in the state of what is called a colloid. 



Substances which are in a "colloidal condition " i.e., 

 are " colloids " are jelly-like, and insoluble in water. 

 They readily absorb water through their substance and 

 swell, while they will also readily yield it up again by 

 evaporation. 



Crystalloids are not merely the reverse of all this, but 

 are specially remarkable for their diffusibility ; while 

 colloids can hardly diffuse themselves at all through the 

 substance of other colloids. 



Substances can often be made to pass from the crys- 

 talloid to the colloidal condition by adding a minute 

 quantity of some substance, such as an alkaline 

 carbonate. As an example of a substance which will 

 alternately exist in these two states, may be mentioned 

 that known as peroxide of iron. 



K 



