COLLOIDS AND CRYSTALLOIDS. 295 



brane upon the two fluids. Prof. Graham, however, believes 

 that the passage or osmose of water through membrane may 

 be explained by supposing that it combines with the membra- 

 nous septum, which thus becomes hydrated, and that on reach- 

 ing the other side it partly leaves the membrane, which thus 

 becomes to a certain degree dehydrated. For example, a 

 membrane such as that used in the endosmometer, is hydrated 

 to a higher degree if placed in pure water than in a neutral 

 saline solution. Hence, in the case of the endosmometer filled 

 with the saline solution and placed in water, the equilibrium 

 of hydration is different on the two sides ; the outer surface 

 being in contact with pure water tends to hydrate itself in a 

 higher degree than the inner surface does. " When the full 

 hydration of the outer surface extends through the thickness 

 of the membrane, and reaches the inner surface, it there re- 

 ceives a check. The degree of hydration is lowered, and 

 water must be given up by the inner layer of the membrane." 

 Thus the osmose or current of water through the membrane is 

 caused. The passage outwards of the saline solution, on the 

 other hand, is not due, probably, to any actual fluid current ; 

 but to a solution of the salt in successive layers of the water 

 contained in the pores of the membrane, until it reaches the 

 outer surface and diffuses in the water there situate. 



Thus, " the water movement in osmose is an affair of hydra- 

 tion and of dehydration in the substance of the membrane or 

 other colloid septum, and the diffusion of the saline solution 

 placed within the osmometer has little or nothing to do with 

 the osmotic result, otherwise than as it affects the state of hy- 

 dration of the septum." 



Prof. Graham has classed various substances according to 

 the degree in which they possess this property of passing, when 

 in a state of solution in water, through membrane ; those which 

 pass freely being termed crystalloids, and those which pass with 

 difficulty, colloids. 



This distinction, however, between colloids and crystalloids, 

 which is made the basis of their classification, is by no means 

 the only difference between them. The colloids, besides the 

 absence of power to assume a crystalline form, are character- 

 ized by their inertness as acids or bases, and feebleness in all 

 ordinary chemical relations. Examples of them are found in 

 albumen, gelatin, starch, hydrated alumina, hydrated silicic 

 acid, &c. ; while the crystalloids are characterized by qualities 

 the reverse of those just mentioned as belonging to colloids. 

 Alcohol, sugar, and ordinary saline substances are examples 

 of crystalloids. 



Absorption by bloodvessels is the consequence of their walls 



