220 Proceedings of Poyal Society of Edinburgh. [may 2 , 
1*1 or 1*2 ... 1*5 times MgO or FeO per 1K 2 0 used, the solution 
filtered hot into a basin, allowed to crystallise, and the crop of 
crystals produced examined. 
The results were similar in both cases, but I prefer to state them 
in reference to the magnesia salt, because our experiments in regard 
to it were more exhaustive than those with the iron salt. 
The crystals deposited from a solution containing one or even 
a little more than one MgO per 1K 2 0 may look perfectly normal 
en masse , but when examined more closely are invariably found 
to be contaminated with a powdery coating which looks like, 
and indeed substantially consists of, sulphate of potash. As the 
proportion of magnesia increases, the relative quantity of free 
sulphate of potash produced gets less and less, and at last vanishes 
altogether. With 1*3, sometimes even with D2 times MgO per 
1K 2 0, we generally obtained normal-looking crystals, which con- 
tained the correct percentage of water, and consequently consisted, 
substantially at least, of the pure, unmixed double salt. 
Similar results were obtained in the case of the ferrous double 
salt ; in its case we determined the percentage of iron (by perman- 
ganate) as a test for the degree of purity. 
In neither case, however, have I been able as yet to determine 
the exact conditions under which a given solution is sure to de- 
posit perfectly normal crystals. Whether or not depends not merely 
on the ratio between the weights of the two bases, but also on the 
proportion of water, the temperature at which the crystals separate 
out, and other independent variables. 
I refrain from giving any further details, intending to complete 
the investigation, to extend it to other double salts of the magnesium 
series, and also to the alums, and then to present a complete and 
detailed memoir. 
5. A Diatomaceons Deposit from North Tolsta, Lewis. 
By John Battray, Esq. 
PRIVATE BUSINESS. 
Mr H. A. Webster, Mr John S. Yeo, Mr John Cockburn, Dr A. 
S. Cumming, and Mr J. W. Capstick were balloted for and declared 
duly elected Fellows of the Society. 
