CAUSES OF THE MOTION OF JUICES. 353 



and have, when dissolved in water or other liquid, a very 

 low capacity for diffusive motion. These bodies are 

 termed Colloids* and are characterized by swelling up or 

 uniting with water to bulky masses (hydrates) of gelati- 

 nous consistence, by inability to crystallize, and by feeble 

 and poorly-defined chemical affinities. Starch, dextrin, 

 the gums, the uncrystallized albuminoids, pectin and pectic 

 acid, gelatin (glue), tannin and gelatinous silica, are col- 

 loids. Opposed to these, in the properties just specified, 

 are those bodies which crystallize, such as saccharose, glu- 

 cose, 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 term- 

 ed by Graham Crystalloids.\ 



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 remains 

 unchanged. 



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 



APPROXIMATE TIMES OF EQUAL DIFFUSION. 



Chlorhydric acid, crystalloid, 1. 

 Chloride of sodium, 2. 



Sugar (cane,) 7. 



Sulphate of magnesia, " 7. 



Albumen, colloid, 49. 



Caramel, " 98. 



* From two Greek words which signify glue-like. 



t We have already employed the word Crystalloid to distinguish the amor- 

 phous albuminoids from their modifications or combinations which present the 

 aspect of crystals, (p. 107.) This use of the word was proposed by N&geli in 

 1802. Graham had employed it, as opposed to colloid, in 18*51. It will perhap3 

 be found that Niigeli's crystalloids are crystalloid in Graham's sense. 



