VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 67 1 



and when the precipitate has at length been formed, it re-dissolves in marine acid 

 as easily as that made by water ; whereas the precipitates resulting from the union 

 of the Prussian matter are not acted on by acids, till that matter has been extracted 

 from them by an alkali. For further satisfaction in this important point, the 

 experiment was repeated with a solution in aqua regia. Here the Prussian lixi- 

 vium, in whatever quantity it was added, occasioned no precipitation at all, only 

 the usual bluishness arising from the iron always found in the common acids ; 

 and pure alkali, added afterwards, precipitated the original white substance un- 

 changed. 



The following experiments of precipitation by alkalis were made with the 

 marine solution, before the effect of an addition of nitrous acid had been dis- 

 covered ; and they were made with so much care and attention, that it was not 

 thought necessary to repeat them afterwards. To obviate, as much as possible, 

 the equivocal results that might arise from water contained in the precipitants, 

 the different alkalis were applied in the dryest state they could be reduced to ; 

 viz. pure salt of tartar, kept for some time in a heat just below redness ; crystals 

 of marine alkali, melted and dried in the same manner ; volatile alkali in crystals, 

 a little surplus acid being, in this instance, previously added to the solution, to 

 counteract the water of crystallization in the alkali ; salt of tartar causticated by 

 quick-lime, and hastily evaporated to dryness ; the marine alkali causticated in 

 like manner ; and the vapour of caustic volatile alkali arising, with a very gentle 

 heat, from a retort into a phial containing the solution. All these alkalis oc- 

 casioned copious precipitations. All the precipitates, after washing and drying, 

 were found to re-dissolve in marine acid ; and from all these solutions the origi- 

 nal substance was precipitated, unaltered, on diluting them with water. 



In strong fire, from 142 to 15()^ this substance discovers a much greater 

 fusibility than any of the known simple earths. In a small vessel, made of to- 

 bacco-pipe clay, it melted, and glazed the bottom ; and on a bed of powdered 

 flint, pressed smooth in the manner of a cupel, it did the same. Magnesia, 

 or chalk, would indeed vitrify in the clay vessel ; but on flint, no one of the 

 known earths shows any tendency to vitrification in that heat.* In a cavity, 

 scooped in a lump of chalk, this substance, in the heat above mentioned, ran 



* It may be proper just to mention, that I find this to be a very commodious and sure method of 

 trying^ in small, whether any given earthy body be fusible with other earths. If the body is disposed 

 to vitrify with any proportion of clay or flint, for instance, it will equally vitrify when a little of it is 

 applied, or even dusted only, on the bottom of a small cup made of clay, or on a smooth close bed of 

 finely powdered flint. The body, in this mode of application, seems to unite with only just so much 

 of the matter of the substraium as i» requisite for their most perfect fiision together, and has nothing 

 else in contact w ith it, so that no deception ca.i arise ; whereas, if mixed with the same matter, there 

 might be no appearance of fusion, unless certain favourable {iroportions of the two should chance to 

 be hit upon ; and even then, if the (juaniiiy be small, it would not be certain but that the fiision 

 might have originated tiora the matter ot the crucible. — Orig. 



