204 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
was now changed in each experiment. The substance in solution amounted to 2 grammes, 
the depth of fluid in the dialyser to 10 millimetres (0'4 of an inch), and the surface of 
the septum to O'Ol square metre (15’6 square inches). 
Table XIV. — Dialysis through Parchment-paper during twenty-four hours, at 12°. 
Two per cent, solutions. 
Diffusate, 
in grammes. 
Proportional 
diffusate. 
Chloride of sodium 
1-657 
1 
Picric acid 
1-690 
1-020 
Ammonia 
1-404 
-847 
1'hein 
1-166 
-703 
Salicin 
-835 
-503 
Cane-sugar 
-783 
-472 
Amygdalin 
-517 
-311 
Extract of quercitron 
-305 
-184 
Extract of logwood 
-280 
-168 
Catechu 
-265 
-159 
Extract of cochineal 
-086 
-051 
Gallo-tannic acid 
-050 
-030 
Extract of litmus 
-033 
-019 
Purified caramel 
-009 
-005 
Picric acid and thein were actually diffused from 1 per cent, solutions, and the num- 
bers observed are multiplied by 2. The crystallizable principles, thein, salicin, and 
amygdalin, appear greatly more diffusible than gallo-tannic acid, or than gum, as has 
been already seen. Such inequality of rate is likely to facilitate the separation of 
vegetable prmciples by the agency of dialysis. 
4. Preparation of Colloid Substances by Dialysis. 
The purification of many colloid substances may be effected with great advantage by 
placing them on the dialyser. Accompanying crystalloids are eliminated, and the 
colloid is left behind in a state of purity. The purification of soluble colloids can rarely 
be effected by any other known means, and dialysis is evidently the appropriate mode of 
preparing such substances free from crystalloids. 
Soluble Silicic Acid . — A solution of silica is obtained by pouring silicate of soda into 
diluted hydrochloric acid, the acid being maintained in large excess. But in addition 
to hydrochloric acid, such a solution contains chloride of sodium, a salt which causes the 
silica to gelatinize when the solution is heated, and otherwise modifies its properties. 
Now such soluble silica, placed for twenty-four hours in a dialyser of parchment-paper, 
to the usual depth of 10 millimetres, was found to lose in that time 5 per cent, of its 
silicic acid, and 86 per cent, of its hydrochloric acid. After four days on the dialyser, 
the liquid ceased to be disturbed by nitrate of silver. All the chlorides were gone, 
with no further loss of silica. In another experiment 112 grammes of silicate of soda, 
67 ’2 grammes of dry hydrochloric acid, and 1000 cub. cents, of water were brought 
together, and the solution placed upon a hoop dialyser, 10 inches in diameter. After 
four days the solution had increased to 1235 cub. cents., by the action of osmose ; colloid 
