COLLOIDAL MIXTURES. 271 



gelatinization, and their property of permitting the free passage 

 through them of non-colloidal substances, but preventing entirely that 

 of other colloids of either of the two classes. This last property is the 

 same one that is involved in the permeability of animal membranes 

 for crystalloids and their impermeability for colloids, since such mem- 

 branes are themselves nothing more than gelatinized colloids. Yet it 

 deserves, on account of its great importance, a somewhat fuller con- 

 sideration. This difference in behavior towards crystalloids and col- 

 loids may be readily illustrated by immersing sticks of gelatine or agar 

 jelly in one experiment in a colored salt solution and in another in a 

 colored colloidal suspension, and allowing them to remain for a day 

 or more. Such comparative experiments may be made in a striking 

 way with a solution of .copper sulphate and ammonia and with a col- 

 loidal suspension of ferric ferrocyanide or Prussian blue made by 

 mixing equal volumes of dilute solutions of ferric chloride and potas- 

 sium ferrocyanide. Upon removing the sticks after some hours and 

 cutting them in two, it will be noticed that the ammoniated copper 

 sulphate has permeated the stick uniformly to its center, while the 

 Prussian blue has not entered it at all. 



Not only are gelatinized colloids permeable to salts, but, remark- 

 ably enough, they offer only a very slight, often scarcely appreciable, 

 hindrance to the passage of these substances through them. Thus, 

 accurate experiments have shown that the rate of diffusion of salts and 

 mineral acids is the same, at any rate within one per cent., in a solid 

 jelly containing 3 to 5 per cent, of agar-agar as it is in one containing 

 only 1 per cent, of agar-agar, and it is, therefore, presumably the same 

 as in water itself, though the accuracy with which this latter conclusion 

 has yet been directly tested is much less. It has also been shown that 

 the electrical conductivity of salts in a gelatine jelly is only a few 

 per cent, different from that in pure water, and that there is no sudden 

 change in its value when the jelly sets. This property is, however, 

 dependent on the rate of motion of the ionized molecules of the salt 

 through the medium between the electrodes, and the slight variation 

 in it caused by the presence of colloids, even in quantity sufficient to 

 produce gelatinization, proves that the flow of such molecules is but 

 little impeded by the colloid. 



Eeturning now for a moment to the other side of the phenomenon 

 — the impermeability of one colloid by another — attention may be called 

 to an apparently related fact of much importance, namely, to the fact 

 that the presence of a gelatinizing colloid in a liquid in fairly small 

 quantity prevents the coagulation of colloidal suspensions by salts, and, 

 therefore, usually prevents the formation of a coagulated precipitate 

 when the solutions of two chemical substances are mixed which, under 

 ordinary conditions, give rise to such a precipitate. Thus when aque- 



