184 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
such colloids as can exist at a high temperature. Hence a wide sensibility on the part 
of colloids to external agents. Another and eminently characteristic quality of colloids, 
is their mutability. Their existence is a continued metastasis. A colloid may be com- 
pared in this respect to water while existing liquid at a temperature under its usual 
freezing-point, or to a supersaturated saline solution. Fluid colloids appear to have 
always specious* modification; and they often pass under the slightest infiuences from 
the first into the second condition. The solution of hydrated silicic acid, for instance, 
is easily obtained in a state of purity, but it cannot be preserved. It may remain fiuid 
for days or weeks in a sealed tube, but is sure to gelatinize and become insoluble at 
last. Nor does the change of this colloid appear to stop at that point. For the mineral 
forms of silicic acid, deposited from water, such as flint, are often found to have passed, 
during the geological ages of their existence, from the vitreous or colloidal into the 
crystalline condition (H. Eose). The colloidal is, in fact, a dynamical state of matter ; 
the crystalloidal being the statical condition. The colloid possesses exeegia. It may be 
looked upon as the probable primary source of the force appearing in the phenomena 
of \itality. To the gradual manner in which colloidal changes take place (for they 
always demand time as an element), may the characteristic protraction of chemico- 
organic changes also be referred. 
A simple and easily applicable mode of effecting a diflusive separation is to place the 
mixed substance under a column of water, contained in a cylindrical glass jar of o or 6 
inches in depth. The mixed solution maybe conducted to the bottom of the jar by the 
use of a fine pipette, without the occurrence of any sensible intermixture. The spon- 
taneous diffusion, which immediately commences, is alloAved to go on for a period of 
several days. It is then interrupted by siphoning off the water from the surface in 
successive strata, from the top to the bottom of the column. A species of cohobation 
has been the consequence of unequal diffusion, the most rapidly diffusive substance 
being isolated more and more as it ascended. The higher the water column, suffi- 
cient time being always given to enable the most diffusive substance to appear at the 
summit, the more completely does a portion of that substance free itself from such other 
less diffusive substances as were originally associated with it. A marked effect is pro- 
duced even where the difference in diffusibility is by no means considerable, such as the 
separation of chloride of potassium from chloride of sodium, of which the relative 
'diffusibilities are as I to 0-841. Supposing a third metal of the potassium group to 
exist, standing above potassium in diffusibility as potassium stands above sodium, it may 
be safely predicated that the new metal would admit of being separated from the other 
two metals by an application of the jar-diffusion above described. 
A certain property of colloid substances comes into play most opportunely in assisting 
difiusive separations. The jelly of starch, that of animal mucus, of pectin, of the vege- 
* Urj/cros, curdled. As fibrin, casein, albumen. But certain liquid colloid substances are capable of 
forming a jelly and yet still remain liquefiable by beat and soluble in water. Such is gelatine itself, which 
is not pectous in the condition of animal jelly ; but may be so as it exists in the gelatiferous tissues. 
