CHEMICAL SCIENCE. 201 



addition of crystalloids. The solution of colloids has always a cer- 

 tain degree of viscosity or gmnminess, when concentrated. They ap- 

 pear to be insipid, or wholly tasteless, unless when they undergo de- 

 composition on the palate, and give rise to sapid crystalloids. Their 

 solid hydrates are gelatinous bodies. They are united to water with 

 a force of less intensity ; and such is the character of the combina- 

 tions in general between a colloid and a crystalloid, even although 

 the latter may be a powerful reagent in its own class, such as an acid 

 or an alkali. In their chemical reactions the crystalloidal appears 

 the energetic form, and the colloidal the inert, form of matter. The 

 combining equivalent of the colloid appears always to be high, and it 

 has a heavy molecule. Among the colloids rank hydrated silicic acid, 

 and a number of soluble hydrated metallic peroxides, of which little 

 has hitherto been known ; also starch, the vegetable gums, and dex- 

 trin, caramel, tannin, albumen, and vegetable and animal extractive 

 matter. The peculiar structure and chemical indifference of colloids 

 appear to adapt them for the animal organization, of which they be- 

 come the plastic elements. Although the two classes are widely sep- 

 arated in their properties, a complete parallelism appears to hold 

 between them. Their existence in nature seems to call for a corre- 

 sponding division of chemistry into a crystalloid and a colloid depart- 

 ment. 



Although chemically inert in the ordinary sense, colloids possess a 

 comparative activity of their own, arising out of their physical prop- 

 erties. While the rigidity of the crystalline structure shuts out ex- 

 ternal impressions, the softness of the gelatinous colloid partakes 

 of fluidity, and enables the colloid to become a medium for liquid 

 diffusion, like water itself. The same penetrability appears to take 

 the form of a capacity for cementation in such colloids as can exist 

 at a high temperature. Hence a wide sensibility on the part of col- 

 loids to external agents. Another eminently characteristic quality 

 of colloids is their mutability. Their existence is a continued met- 

 astasis. A colloid may be compared in this respect to water while 

 existing liquid at a temperature below its usual freezing point, or to 

 a supernatural saline solution. The solution of hydrated silicic acid, 

 for instance, is easily obtained in a state of purity, but cannot be pre- 

 served. It may remain fluid for days or weeks in a sealed tube, but it 

 is sure to gelatinize at last. Nor does the change of this colloid stop 

 at that point; for the mineral forms of silicic acid deposited from 

 water, such as flint, are found to have passed, during the geological 

 ages of their existence, from the vitreous or colloidal into a crystalline 

 condition. The colloidal is in fact a dynamical state of matter ; the 

 crystalloidal being the statical condition. 



The separation of a crystalloid from a colloid is readily effected 

 by a combination of diffusion with the action of a septum composed 

 of an insoluble colloidal material. Animal membrane will serve for 

 the latter purpose, or a film of gelatinous starch, hydrated gelatine 

 itself, albumen, or animal mucus. But much the most effective sep- 

 tum used is paper as it is metamorphosed by sulphuric acid. This is 

 now supplied by Messrs. De la Rue, and has become familiar under 

 the name of "vegetable parchment," or "parchment paper." From 

 sheet gutta-percha a flat hoop is formed, eight or ten inches in diame- 



