Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/248

 sulphat of iron and earthy salts, and dissolve all the magnesia which might be present. This process was, of course, attended with considerable effervescence, and when this had subsided, the liquor was filtered. The clear solution deposited on standing a brownish sediment, which was separated and proved to be oxyd of iron. The residue left in the filter had passed from a greenish-yellow to a pale brown colour.

2. Phosphat of ammonia being added to the clear solution, a precipitate appeared, having all the characters of the ammoniaco-magnesian phosphat, and in particular, that of forming white stripes on the inside of the vessel when scratched with a pointed instrument. This precipitate dried at a temperature of about 120°, weighed 1,9 grains, and being made red hot in a platina crucible, was reduced to exactly 1 gr. = 0,385 grains of pure magnesia = 2,26 grains of crystallized sulphat of magnesia in 50 grains of residue, or 3,63 grains in a pint of the water It will be necessary here to state the grounds of this computation, which will afford me an opportunity of relating some general results concerning the proportions in which magnesia and phosphoric acid combine.

By dissolving 11,82 grains of the purest magnesia (perfectly free from carbonic acid and water) in muriatic acid, and precipitating it by a mixture of phosphat of ammonia and neutral carbonat of ammonia, I obtained 65,8 grains of the triple phosphat dried by exposure for near forty-eight hours to a temperature which never exceeded 120°, a degree of heat under which this salt appears to retain the whole of its ammonia These 65,8 grains of triple salt being exposed for half an hour to a strong red heat in a platina crucible, were reduced to 30,8 grains. The salt appeared then in the form of a friable cake or loose aggregate, a fragment of which, on being urged by the blow-pipe, run into a white opaque vitreous globule, without any further diminution of weight. In its friable state it was readily dissolved by muriatic acid; in its vitrified form it required heat and trituration. This salt was perfectly tasteless and shewed no attraction for water. With regard to the proportions of acid and base to be inferred from this experiment, it is obvious that if 80,8 grains of phosphat of magnesia contain 11,82 grs. of earth, the remainder, viz. 18,98 grs. represents the proportion of phosphoric acid; which is equivalent to 38,37 grs. of magnesia, in 100 of phosphat. In another experiment conducted in a similar manner, the magnesia amounted to 38,7 grains, so that by taking the mean between these two very nearly similar results, we have the following proportions, viz.

We may infer therefore that one grain of phosphat of magnesia, the quantity yielded by the twenty grains of residue, indicated 0,385 of pure magnesia; and if, according to the statements of Kirwan and Wenzel (which very nearly agree) one hundred grains of crystallized sulphat of magnesia contain seventeen grains of magnesia, 2,26 grains of that salt will be the quantity corresponding to 0,385 grains of magnesia. And I have the satisfaction of observing that the proportions obtained by Dr. Henry, of one hundred grains of ammoniaco-magnesian phosphat dried at 90°, for one hundred and eleven grains of crystallized sulphat of magnesia, would have led to a very similar result. (See Dr. Henry's Analysis of several varieties of Salt, in Philos. Trans. for 1810, page 113.) The magnesia