250 ANNUAL OF SCIENTIFIC DISCOVERY. 



salt be placed in the dialyser, the salt rapidly diffuses itself into the 

 water in the outer vessel, and the solution of pure silica in water 

 remains in the dialyser. This solution is found to have a feebly acid 

 reaction on test paper, but not to the taste, as, being a colloid, it can- 

 not pass through the membrane of the tongue so as to affect the 

 nerves of taste. The solution of silica remains for some time perfectly 

 limpid, but eventually sets into a firm jelly. This alteration may be 

 brought about immediately by the presence of several substances, par- 

 ticularly by any earthy carbonate such as chalk. This solution of 

 pure silica possesses remarkable properties ; it is absorbed by gelatin- 

 ous tissues such as the skin of animals, in the same manner as tannin ; 

 and like it converts them into a kind of leather, which possesses the 

 remarkable property of not putrefying when kept moist. In the 

 same manner a solution of pure peroxyd of iron may be obtained, by 

 first dissolving excess of the hydrated oxyd in hydrochloric acid", 

 and then dialysing, when a colloid solution of oxyd of iron remains, 

 that is capable of being gelatinized like the silica. Prussian blue, 

 which is insoluble in pure water, is capable when recently precipi- 

 tated, of being dissolved by the aid of gentle heat in a solution of ^ 

 of its weight of oxalic acid, when it forms the well-known permanent 

 blue ink. If such a solution be dialysed. the Prussian blue is, in the 

 course of a few days, obtained in a solution in pure water, and may 

 be rendered gelatinous by the addition of sulphate of zinc and several 

 other metallic salts, as the solution of silica is gelatinized by the addi- 

 tion of carbonate of lime. 



Such are a few of the many examples of these remarkable phenom- 

 ena. They are as yet of too recent discovery to have been applied to 

 many practical purposes but a vast number of applications at once 

 suggest themselves. In cases of the suspected poisoning of articles 

 of food, the poison, if a crystalloid substance like arsenic, can be 

 readily dialysed and obtained in a pure form, however heterogeneous 

 may be the mixture in which it is contained. Dyeing will be greatly 

 facilitated by steeping a fabric in a pure solution of some colloid dye, 

 which will unite with the animal or vegetable fibre as it gelatinizes. 



The purification of many drugs and the separation of different sub- 

 stances in the chemical arts will be rendered much easier than hereto- 

 fore. In fact, there appears scarcely a limit to the application of this 

 principle. Already, dialysis has thrown light upon obscure points in 

 geology, such as the formation of Hints and other silicious fossils, and 

 it promises equally to benefit physiological research. In fact, humble 

 and inconspicuous as its phenomena may appear at first sight, it is 

 probable that, in its inlluence on science and art, it will greatly sur- 

 pass any discovery of late years. 



