‘^9 
PROFESSOR GRAHAM ON THE DIFFUSION OF LIQUIDS. 
solved separately, in the proportions in which they existed in the double salt, namely, 
1'65 gr. anhydrous sulphate of magnesia in 100 water, and 2*35 grs. sulphate of pot- 
ash in 100 water, making up together 4 parts of salts. The two solutions thus con- 
tain equivalent quantities of the different sulphates. 
The separate diffusion of the sulphate of magnesia was 2*09, 2*11 and 2*40 grs. in 
three cells; and of the sulphate of potash, 5'83, 5 97 and 5‘54 grs. in three ceils; the 
circumstances of the experiments being the same as those of the double salt. The 
means of the two salts are 2'20 and 5'78 grs. ; and the sum of the two means 7‘98 grs. 
The result is, that the separate diffusion of the constituent salts is almost identical 
with their diffusion when combined as a double salt : — 
Diffusion of the double sulphate of magnesia and potash . . . 7*95 grs. 
Diffusion of equivalents of sulphate of magnesia and sulphate of") 
potash in separate cells J 
It would thus appear that the diffusibility of this double salt is the sum of the 
separate diffusions of its constituent salts. 
It has been a question whether a double salt is formed at once when its constituent 
salts are dissolved together, or not till the act of crystallization of the compound salt. 
Equivalents of the same two sulphates, making up 4 parts, were dissolved together 
without heat in 100 water. Now the diffusion from this mixture, which has the com- 
position of the preceding solution of the double salt, exhibited notwithstanding a 
sensibly different result of diffusion, giving 7‘28, 7'37 and 7'26 grs. in three cells 
mean, 7'30 grs. The diffusion of the double salt was greater, namely, 7'95 grs. 
Hence a strong presumption that the mixed salts last diffused were not combined, and 
that the double sulphate of magnesia and potash is not necessarily formed imme- 
diately upon dissolving together its constituent salts. 
In early experiments of a similar nature made upon the double salt, sulphate of 
copper and potash, and upon a mixture of the two sulphates newly dissolved together, 
a similar result was obtained. While the diffusion of the mixed salts was 25’6grs., 
that of the same weight of the combined salts (the double sulphate) was 30 grs. The 
double salt appears more diffusible, in both cases, than its mixed constituents. 
These double salts appear to dissolve in water without decomposition, although the 
single salts may meet in solution without combining. Hence in a mixture of salts 
we may have more than one state of equilibrium possible. And when a salt, like 
alum, happens to be dissolved in such a way as to decompose it, the constituents are 
not necessarily reunited by subsequent mixing. Many practices in the chemical arts, 
which seem empirical, have their foundation possibly in facts of this kind. 
8. Diffusion of one Salt into the Solution of another Salt. 
It was curious and peculiarly important, in reference to the relation of liquid to 
gaseous diffusion, to find whether one salt A would diffuse into water already charged 
