COLLOIDS AND CRYSTALLOIDS. 369 



certain degree de-hydrated. For example, a membrane 

 such as that used in the endosinometer, is hydrated to a 

 higher degree if placed in pure water than in a neutral 

 saline solution. Hence, in the case of the endosinometer 

 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 ex- 

 tends through the thickness of the membrane, and reaches 

 the inner surface, it there receives 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 con- 

 tained 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 

 hydration and of de-hydration 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 hydration 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 crystal- 

 loids 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 crystal- 

 line form, are characterised by their inertness as acids or 

 bases, and feebleness in all ordinary chemical relations. 

 Examples of them are found in albumen, gelatin, starch, 



