222 
ME. T. GEAHAM ON LIQUID DIPPUSION APPLIED TO ANALYSIS. 
the colloid by a degree of softness, with a more or less rounded outline. The water 
of crystallization is represented by the water of gelatination. The water in gelatinous 
hydrates is aptly described by M. Chevreul as retained by “ capillary affinity,” that is, 
by an attraction partaking both of the physical and chemical character. While it is 
here admitted that chemical affinity of the lowest degree may shade into capillary at- 
traction, it is believed that the character of gelatinous hydration is as truly chemical as 
that of crystalline hydration. Combination of a colloid with water is feeble, it is true, 
but so is combination in general with the colloid. Notwithstanding this, anhydrous 
colloids can decompose certain crystalloid hydrates. The water in alcohol of greater 
strength than corresponds with the density 0-926, which represents the definite hydrate 
C 4 Hg O 2 + 6 HO, is certainly in a state of chemical union. But alcohol so high as 0-906, 
contained in a close vessel, is concentrated in a notable degree by contact with dry mucus, 
gelatine, and gum, and sensibly even by dry parchment-paper. Dilute alcohol divided 
from the air of the atmosphere by a dry septum of mucus, gelatine, or gum, is also con- 
centrated by evaporation, as in the well-known bladder experiment of Sommeking. The 
selective power is here apparent of the colloid for Avater, that fiuid being separated from 
alcohol, and travelling through the colloidal septum by combination with successive 
molecules of the latter, till the outer surface is reached and evaporation takes place. 
The penetration in this manner of a colloid by a foreign substance may be taken as an 
illustration of the phenomena of cementation. Iron and other substances which soften - 
under heat, may be supposed to assume at the same time a colloidal constitution. So it 
may be supposed does silica Avhen fused into a glass by heat, and every other vitreous 
substance. 
Gelatinous hydrates ahvays exhibit a certain tendency to aggregation, as is seen in the 
jelly of hydrated silicic acid and of alumina. With some the jelly is also adhesive, a« in 
glue and mucus. But unless they be soluble in water, gelatinous hydrates, when once 
formed, are not in general adhesive. Separated masses do not reunite when brought into 
contact. This Avant of adhesiveness is very remarkable in the gelose of Pa yen, which 
resembles gelatine so closely in other respects. Layers of a gelose solution, allowed to 
cool and gelatinize in succession in a diffusion-jar (p. 199), do not adhere together. 
Ice itself presents colloidal characters at or near its melting-point, paradoxical although 
the statement may appear. When ice is formed at temperatures a feAV degrees under 
O'* C., it has a well-marked crystalline structure, as is seen in Avater frozen from a 
state of vapour, in the form of flakes of snoAV and hoar-frost, or in water frozen from 
dilute sulphuric acid, as observed by Mr. Faeaday. But ice formed in contact with 
water at 0°, is a plain homogeneous mass with a vitreous fracture, exhibiting no facets 
or angles. This must appear singular when it is considered how favourable to crystal- 
lization are the ch’cumstances in which a sheet of ice is slowly produced in the freezing 
of a lake or riA-er. The continued extrication of latent heat by ice as it is cooled a few 
degrees beloAV 0° C., obserA'ed by M. Person, appears also to indicate a molecular change 
subsequent to the first freezing. Further, ice, although exhibiting none of the viscous 
