VOL.'LXXX.] PHILOSOPHICAL TRANSACTIONS. 669 



In all the experiments of dissolution, as often as the heat was at or near the 

 boiling point of the acid, frequent and pretty singular bursts or explosions hap- 

 pened, though the matter lay very thin in a broad-bottomed glass. They were 

 sometimes so considerable as to throw off a porcelain cup with which the glass 

 was covered, and once to shatter the glass in pieces. In a heat a little below this, 

 the extraction seemed to be equally complete, though more slow; but a heat a 

 little below that in which wax melts, or below 140^ of Fahrenheit's thermometer, 

 appeared insufficient. 



To determine the degree of dilution necessary for the precipitation of the dis- 

 solved substance, and whether the precipitation by water be total, a measure of 

 the solution was poured into a large glass, and the same measure of water added 

 repeatedly. The 3d addition of water occasioned a slight milkiness, which in- 

 creased more and more to the 6th. The liquor being then filtered off, another 

 measure of water produced a little fresh milkiness , and an 8th rather increased 

 it ; a Qth and a 10th had no effect. The liquor being now again passed through 

 a filter, solution of salt of tartar did not in the least alter its transparency; so that, 

 after the solution has been diluted with 8 or Q times its measure of water, there is 

 nothing left in it that alkali can precipitate. 



From the manner in which the solution is necessarily prepared, it cannot but 

 contain a great redundance of acid; for the small quantity of acid, sufficient for 

 holding the soluble part suspended, would be soaked up or entangled by the un- 

 dissolved part, so as scarcely to admit of any being poured off; and it cannot be 

 diluted, or washed out, but by the strong acid itself. The solution with which 

 the above experiment was made was reckoned to have only about 6 grs. of the 

 soluble matter to 3 oz. of spirit of salt, having been prepared by digesting that 

 quantity of the spirit by half an ounce at a time on 30 grs. of the crude mineral. 

 A saturated solution was obtained by digesting, in a small portion of the solutions 

 thus prepared, the precipitate thrown down by water from the larger portions, 

 till the acid would take up no more. A solution thus saturated cannot bear the 

 smallest quantity of water, a single drop, on the first contact, producing a milky 

 circle round it. 



This substance, washed and dried, is indissoluble in water, as indeed might be 

 expected from the manner of its preparation. Nor is it acted on by the nitrous 

 or vitriolic acids, concentrated or diluted, cold or hot ; nor by alkaline solutions, 

 mild or caustic, of the volatile or fixed kind. It is dissolved by strong marine 

 acid, but not without the assistance of nearly thjs same degree of heat that is 

 necessary for its extraction from the mineral. From this solution it is precipi- 

 tated by water ; and, after repeated dissolutions and precipitations, it appears to 

 have suffered no decomposition or change. 



Spirit of nitre, added to the saturated solution, makes no precipitation ; and if 



