393 HOW CROPS GROW. 



Colloids and Crystalloids. — There is a class of bodies 

 whose molecules are singularly inactive in many respects, 

 and hare, when dissolved in water or other liquid, a 

 very low capacity for diffusive motion. These bodies 

 are termed Colloids,* and are characterized hy swelling 

 up or uniting with water to bulky masses (hydrates) of 

 gelatinous consistence, by inability to crystallize, and by 

 feeble and poorly-defined chemical affinities. Starch, 

 dextrin, the gums, the albuminoids, pectin and pectic 

 acid, gelatin (glue), tannin, the hydroxides of iron, and 

 aluminium and gelatinous silica, are colloids. Opposed 

 to these, in the properties just specified, are those bodies 

 which crystallize, such as saccharose, glucose, oxalic, 

 citric, and tartaric acids, and the ordinary salts. 



Other bodies which have never been seen to crystallize 

 have the same high diffusive rate ; hence the class is 

 termed by Graham Crystalloids, f 



Colloidal bodies, when insoluble, are capable of imbib- 

 ing liquids, and admit of liquid diffusion through their 

 molecular interspaces. Insoluble crystalloids are, on 

 the other hand, impenetrable to liquids in this sense. 

 The colloids swell up more or less, often to a great bulk, 

 from absorbing a liquid ; the volume of a crystalloid 

 admits of no such change. 



In his study of the rates of diffusion of various sub- 

 stances, dissolved in water to the extent of one per cent 

 of the liquid, Graham found the following 



^ APPKOXIMATE TIMES OF EQUAL DIFFUSION. 



Hydrochloric acid, CrystaUoid, 1. 



* From two Greek words which signify glue-like. 



t We have already employed the word Crystalloid to distinguish the 

 amorphous albuminoids froi^ their modiflcations or combinations 

 which present the aspect of crystals (p. 107). This use of the word was 

 proposed by Nagell, In 1862. Graham had emploved It, as opposed to 

 colloid, In 1861. 



