4. On the Instability of the Double Sulphates M″SO₄.R′₂SO₄ + 6H₂O of the Magnesium Series

Extract

By a number of observations made incidentally in the preparation of two of the double salts referred to in the heading, namely, the compounds of sulphate of potash with sulphate of magnesia and sulphate of ferrous oxide respectively, I had long come to suspect that these two salts at any rate are not perfectly stable in opposition to water. To settle the question, I have caused Mr James Eobson and Mr Andrew Hodge, two young chemists working in my laboratory, to inquire into the matter by systematic experiments. These were, in general at least, conducted according to the following scheme: —Starting from a known weight of sulphate of potash, this was dissolved in a proportion of hot water,* less than sufficient to hold the intended double salt in solution after cooling; there was then added a known weight of the di-valent sulphate amounting to exactly 1 or 1·1 or 1·2 … 1·5 times MgO or FeO per 1K₂O used, the solution filtered hot into a basin, allowed to crystallise, and the crop of crystals produced examined.