4 GENERAL COXSIDERATIONS ON THE TEXTURES. 



into two classes — the cyi/staUoid and the colloid — which differ in several important 

 characters. Crystalloid bodies, of which water, most salts and acids, and sugar, 

 may be taken as examples, have a disposition to assume a crystalline state ; their 

 solutions are usually sapid, diffluent, and free from viscosity ; they readily diffuse 

 in liquids, and jjass through moist organic membranes or artificial septa of 

 organic matter, such as parchment-paper. Colloids, on the other hand, are 

 characterised by low diffusibility and great indisposition to permeate organic 

 septa, so that when they are associated with crystalloids, the latter may be easily 

 separated by diffusion through a septum into another fluid ; i.e., by " dialysis." 

 Colloids are, moreover, generally tasteless ; they have little or no tendency to 

 crystallize, and then* solution, when concentrated, is always, in a certain degree, 

 viscous or gummy. Among the colloids may be reckoned hydrated silicic acid, 

 and various hydrated metallic peroxides, also albumin, gelatin, starch, gnim, 

 and vegetable and animal extractive matters. Several substances may exist 

 either in the colloid or the crystalloid condition. In point of clu-ntical activity 

 the crystalloid appears to be the more energetic, and the colloidal the more inert 

 form of matter ; but the colloids possess an activity of their own. arising out of 

 then- physical proiserties, and especially then- penetrability, by which the}' become 

 a medium for liquid diffusion, like water itself. Another characteristic is their 

 tendency to change ; the solution of hydrated silicic acid, for instance, cannot 

 be preserved ; after a time it congeals. In this respect a liquid colloid might be 

 compared to liquid water at a temperature below freezing, or to a supersaturated 

 saline solution. This dominant tendency of the particles of a colloid to cohere, 

 aggregate, and contract, is obvious in the gi-adual thickening of the liquid and 

 its conversion into a jelly ; and in the jelly itself the contraction still proceeds, 

 causing separation of water, and division into a clot and semm. Their permea- 

 bility to fluids, their ready capability of physical changes, and their comparative 

 chemical inertness, are properties by which colloid bodies seem fitted to form 

 organised structures, and to take part in the processes of the living economy. 

 Graham further found that silicic acid may combine both in a dissolved 

 and m a gelatinous state with a variety of very different fluids without 

 imdergoing alteration ; and presuming that the organic colloids are invested 

 with similar wide powers of combination, he remarks that the capacity of 

 a mass of gelatinous silicic acid to assume alcohol, or e-ven olein, without 

 disintegration or alteration of form, and to yield it up again m favoLU' of some 

 other substituted fluid, may perhaps afford a clue to the penetration of the colloid 

 matter of animal membrane by fatty and other bodies insoluble m water : and 

 moreover, that the existence of fu'ul compounds of silicic acid of a like nature, 

 suggests the possibility of the fonnation of a compound of colloid albumen with 

 olein, solul3le also and capable of circulatmg with the blood.* 



The important relation which this chemical doctrine bears to the constitu- 

 tion and organic processes of the animal body, has appeared to justify the 

 introduction of the present notice of it ; for further information the reader is 

 referred to the sources aheady cited. 



VITAL PROPERTIES OF THE TEXTURES. 



Of the phenomena exhibited by living bodies, there are some which, 

 in the present state of knowledge, cannot be referred to the operation 

 of any of the forces which manifest themselves in inorganic nature ; 

 they are therefore ascribed to certain powers, endowments, or properties, 

 which so far as known, are peculiar to living bodies, and are accord- 

 ingly named " vital properties." These vital properties are called into 

 play by various stimuli, external and internal, physical, chemical, and 

 mental ; and the assemblage of actions thence resulting has been 

 designated by the tem " life." The words " life " and " vitality " are 



* On the Properties of Silicic Acid and other Analogous Colloidal Substances,— Pro- 

 ceedmgs of the Royal Society, .June 16th, 18(34. 



