PROFESSOR GRAHAM ON THE DIFFUSION OF LIQUIDS. 
19 
exists in the one form and how much in the other. Knowing- however the different 
rates of diffusibility of these two salts, which is nearly chloride 2 and sulphate 1, and 
their relation to the diffusibility of chloride of sodium, we should be able to judge from 
the proportion in which the magnesia travels in company with chloride of sodium, 
whether it is travelling in the large proportion of chloride of magnesium, in the small 
proportion of sulphate of magnesia, or in the intermediate proportion of a certain 
mixture of chloride and sulphate of magnesia. But here we are met by a difficulty. 
Do the chloride of magnesium and sulphate of magnesia necessarily pre-exist in sea- 
water in the proportions in which they are found to diffuse ? May not the more easy 
diffusion of chlorides determine their formation in the diffusive act, just as evapora- 
tion determines the formation of a volatile salt — producing carbonate of ammonia, 
for instance, from hydrochlorate of ammonia with carbonate of lime in the same 
solution? We shall see immediately that liquid diffusion, as well as gaseous evapo- 
ration, can produce chemical decompositions. 
6. Decomposition of Salts hy Diffusion. 
(1.) At an early period of the inquiry, a solution was diffused of bisulphate of pot- 
ash, saturated at 68° and of density 1'280, from the six-ounce phial of ri75 inch aper- 
ture, into 20 ounces of water. The period of diffusion extended to fifty days. About 
the middle of that period, a few small crystals of sulphate of potash, amounting pro- 
bably to 3 or 4 hundredths of a grain, appeared in the diffusion cell and never after- 
wards dissolved away. When terminated, the liquid remaining in the solution cell 
was found of density 1*154; that in the water-jar T0326. A portion of the latter 
liquid gave by analysis — 
Sulphate of potash 20*37 ") 
Sulphate of water 11*47/ 
Sulphate of water 12*77 
Bisulphate of potash. 
44*61 
It thus appears that the bisulphate of potash undergoes decomposition in diffusing, 
and that the acid diffuses away to about double the extent, in equivalents, of the sul- 
phate of potash. This greater escape of the acid will also account for the deposition 
of crystals of the neutral sulphate in the solution cell. 
(2.) A similar experiment was made with another double sulphate of greater stabi- 
lity, common potash-alum. The solution of 4 anhydrous alum in 100 water, was 
diffused from the six-ounce phial into 24 ounces of water, at 64°*2, for eight days. 
The quantity of salt diffused in that time amounted only to 7'48 grs. It contained 
1*06 gr. alumina, which is equivalent to 5*33 grs. of alum. The diffused salt gave off 
no acid vapours at 600°. We may therefore suppose the excess of salt which is dif- 
fused to be sulphate of potash. The diffusion product of alum, at 64°, appears to be — 
Alum 5*33 71*26 
Sulphate of potash . . . 2*15 28*74 
D 2 
7*48 
100*00 
