o?8 Mr. T. Graham on Liquid Diffusion applied to A?ialysis. 



hydrates. The water in alcohol of greater strength than corre- 

 sponds with the density 09:26, which represents the definite 

 hydrate C 4 H b 2 + 6HO, 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 concentrated 

 by evaporation, as in the well-known bladder experiment of 

 Sommering. The selective power is here apparent of the 

 colloid for water, that fluid 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 when fused into a glass by heat, and every other 

 vitreous substance. 



Gelatinous hydrates always 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, as 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 want of adhesiveness is very remarkable in the gelose of 

 Payen, 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. 290), 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 few degrees under 0° C., it has 

 a well-marked crystalline structure, as is seen in water frozen 

 from a state of vapour, in the form of flakes of snow and hoar- 

 frost, or in water frozen from dilute sulphuric acid, as observed 

 by Mr. Faraday. 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 con- 

 sidered how favourable to crystallization are the circumstances 

 in which a sheet of ice is slowly produced in the freezing of a 

 lake or river. The continued extrication of latent heat by ice as 

 it is cooled a few degrees below 0° C, observed by M. Person, 

 appears also to indicate a molecular change subsequent to the 

 first freezing. Further, ice, although exhibiting none of the 

 viscous softness of pitch, has the elasticity and tendency to rend 



