The meconic acid group of substances has always been regarded as peculiarly interesting. Situated, as it is, on the ill-defined limits of the fatty and aromatic compounds, and exhibiting interesting isomeric relations with other organic substances, the structure and derivatives of the whole group afford ample scope for theoretical speculation. Meconic acid has always been considered a tribasic acid, which, by the successive elimination of two molecules of carbonic anhydride, yields first a dibasic and then a monobasic acid. But chemists now know that the basicity of an acid may be less than its atomicity, and that even the former cannot be deduced from the composition of one or two isolated salts, but only from a comprehensive view of all the salts and ethers of the acid. As now a general review of the salts and ethers of meconic acid seems to point to an acid with an atomicity greater than its basicity, and as any additional proof of this constitution would help to explain many abnormal decompositions in the series, the authors have investigated the action of the general reagents on these compounds that enable us to distinguish between the basic and alcoholic hydroxyls.
The fragmentary nature of our knowledge of the behaviour of the more strongly basilous hydrates and carbonates at high temperatures is owing chiefly to the absence of a suitable material for the necessary crucibles. Unfortunately there is no metal which combines the infusibility of platinum with the chemical inertness of gold, in opposition to fiery-fluid caustic alkalies. But the corrosive action of these on platinum, as I showed some years ago, is a function only of the peroxides formed from them by the action of atmospheric oxygen, and, consequently, can easily be prevented by operating in an atmosphere of hydrogen or nitrogen.
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.
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