320 
ON POTASH AND SODA IN SOILS. 
uniform) to the action of a sufficient quantity of a saturated 
solution of sulphate of potassa which refused to dissolve the 
minutest portion of sulphate of potassa, when added in the 
finest powder. It was then filtered through two counter- 
poised filters, and washed with a little of the saturated solu- 
tion. The filters were then put between folds of bibulous 
paper, and pressed with a weight, and dried thoroughly by 
a heat just insufficient to char the paper, until there was no 
further loss of weight. The weight of the sulphate of potassa 
cellected on the inner filter (obtained by the difference of 
weight of the two filters,) corresponded exactly with that 
known to exist in the mixture. The weight of the sulphate 
of potassa collected on the inner filter, (obtained by the differ- 
ence of weight of the two filters,) corresponded exactly with 
that known to exist in the mixture. The weight of the sulphate 
of potassa thus obtained, deducted from the weight of the mix- 
ed salts, gave of course that of the sulphate of soda.* I have 
been thus particular in mentioning the manipulation, in or- 
der that the process may succeed equally in other hands than 
my own. Having attained such an exact result in a simple 
mixture of sulphates of alkalies, I was anxious to know whe- 
ther my plan was equally applicable, in combination with 
the ammoniacal phosphate of soda, to the separation of mag- 
nesia: and here I had occasion to notice the interference of 
the formation of a double salt in vitiating the result. My 
first trial was directed to the dissolving out of the mixture of 
sulphates, the sulphates of magnesia and soda ; by the satu- 
rated solution of sulphate of potassa, expecting that the sul- 
phate of potassa would be left intact, as in the former case; but 
I found that it did not weigh much more than half the proper 
quantity. This I repeated more than once with a similar re- 
sult. 
I proceeded, however, to precipitate the magnesia by the 
double phosphate, which gave the quantity correctly. I then 
altered my plan, by dissolving the mixed salt in a small quan- 
♦The quantities of the pure alkalies are to be obtained by calculations 
from the known composition of their sulphate. 
