Mr. T. Graham on Liquid Diffusion applied to Analysis. 305 



Sucrate of Lime. — The well-known solution of lime in sugar 

 forms a solid coagulum when heated. It is probably, at a high 

 temperature, entirely colloidal. The solution obtained on cool- 

 ing passes through the septum, but requires a much longer time 

 than a true crystalloid like the chloride of calcium. 



The blue solution of tartrate of copper in caustic potash con- 

 tains a colloidal compound, which has not been fully examined. 



Soluble Chromic Oxide. — The definite terchloride of chromium, 

 being a crystalloid, diffuses away entirely when placed in solu- 

 tion upon the dialyser. This salt dissolves, with time, a certain 

 portion of freshly-precipitated hydrated chromic oxide, and 

 becomes of a deeper green colour. Such a solution, after dia- 

 lysis for twenty-two days, contained 8 hydrochloric acid to 92 

 chromic oxide; and after thirty-days, 4"3 acid to 95*7 oxide, or 

 1 equiv. acid to 10*6 equivs. oxide. After thirty-eight days, 

 the solution gelatinized in part upon the dialyser, and then con- 

 tained 1*5 acid to 98*5 oxide, or 1 equiv. acid to 31*2 equivs. 

 chromic oxide. This last solution, which maybe taken to repre- 

 sent soluble chromic oxide, is of a dark-green colour, and admits 

 of being heated, and also of being diluted with pure water with- 

 out change. It was gelatinized with the usual facility by traces 

 of salts and other reagents which affect colloid solutions, and 

 was then no longer soluble in water, even with the assistance of 

 heat. It appeared to be the green hydrated oxide of chromium 

 as that substance is usually known. A metachromic oxide may 

 possibly be obtained by heating and dialysing the acetate, but I 

 have not attempted to form it. 



Mr. Ordway succeeded in dissolving an excess of the hydrated 

 uranic oxide and of glucina in the chloride of uranium and of 

 glucinum respectively. The dialysis of such solutions may be 

 reasonably expected to yield soluble uranic oxide and soluble 

 glucina. 



It appears, then, that the hydrated peroxides of the aluminous 

 type, when free, are colloid bodies ; that two species of each of 

 these hydrated oxides exist, of which alumina and metalumina 

 are the types — one derived from an unchanged salt, and the 

 other from the heated acetate of the base ; further, that each of 

 these species has two forms — one soluble, and the other insoluble 

 or coagulated. This last species of duality should be well distin- 

 guished from the preceding allotropic variability of the same 

 peroxide. The possession of a soluble and an insoluble (fluid 

 and pectous) modification is not confined to hydrated silicic acid 

 and the aluminous oxides, but appears to be very general, if not 

 universal, among colloid substances. The double form is typified 

 in the fibrine of blood. 



The precipitated and gelatinous peroxide of tin is largely 



