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 pectio 

 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 nsoluble, 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 le*s, often to a great bulk, from 

 absorbing a liquid : the volume of a crystalloid remains 

 unchanged. 



In his study of the rates vf diffusion of various sub 

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

 of the liquid, Graham found tbv lullowing 



APPROXIMATE TIMES Of ?QUAL DIFFUSION. 



Chlorhydric acid, ystalloid, 1. 



Chloride of sodium, &quot; 2, 



Sugar (cane,) 7. 



Sulphate of magnesia, 7. 



Albumen, volloid, 49. 



Caramel, &quot; 98. 



* From two Greek words which signify glue-like. 



t We have already employed the word CrystaUoid to distingrmVi the amor 

 phous albuiminoids from their modifications or combiPttiTts whic.v ?s.t*ent th.t 

 aspect of crystals, (p. 107.) This use of the word was proposed *T fV^eli in 

 1862. Graham had employed it, as opposed to colloid iSrfl, It w*S 

 be found that Nageli s crystalloids are crystalloid in Grilwi ^ SOLUS. 



