328 Analysis of Scientific Books. 



flieans of its free phosphoric acid ; and the advantage of em- 

 ploying this salt, rather than the acid itself is, that the latter is 

 very deliquescent, much more costly, and is readily absorbed 

 by the cliarcoal. The salt of phosphorus, therefore, shews 

 the action of acids on the assays ; its excess of acid seizes on 

 all their bases, and forms with them more or less fusible double 

 salts, whose transparency and colour may then be examined. 

 Consequently, this flux is more particularly applicable to the ex- 

 amination of metallic oxides, whose characteristic colours it de- 

 velopes much better than borax. 



" The same flux exerts a repellant action on acids. Those 

 which are volatile sublime, whilst the fixed acids remain in the 

 mass, and divide the base with the phosphoric, or yield it en- 

 tirely to it; in the latter event, they remain in suspension in 

 the glass, without dissolving in it. In this respect the salt of 

 phosphorous is a good re-agent for the silicates ; it sets the 

 silica free, which then appears liquefied in the salt as a gela- 

 tinous mass." 



Saltpetre is useful in detecting small portions of manganese. 

 Vitrified boracic acid, in conjunction with iron, for ascertaining 

 the presence of phosphoric acid ; gypsum and fluor spar, are 

 used mutually to detect each other, and have no other use. 

 Nitrate of cobalt in solution in water, is employed to ascertain 

 the presence of alumina and magnesia ; with the former it gives 

 a blue colour, with the latter a pale rose colour. The directions 

 given for applying this test must be carefully attended to, to 

 avoid equivocal results. Tin, in the state of foil, is used to 

 promote the reduction of metallic oxides; iron, to precipitate 

 certain metals, as copper, lead, nickel, and antimony, and to 

 separate them from sulphur, or fixed acids. Lead is employed 

 in cupellation with bone-ashes. 



" The bone ashes must be reduced to a very fine powder, a 

 small quantity of which is to be taken on the point of a knife 

 moistened with the tongue, and kneaded in the left hand, with a 

 very little soda, into a thick paste. A hole is then made in a 

 piece of charcoal, and filled with the paste, and its surface 

 smoothed by pressure with the agate pestle. It is then to be 

 gently heated by the blow-pipe, till it is perfectly dry, (the soda 

 only assists the cohesion, and may be omitted). The assay, 

 previously fused with lead, is placed in the middle of the little 

 cupel, and the whole heated by the exterior flame. When the 

 operation is finished, the precious metals are left on the surface 

 of the cupel. This experiment is so delicate, that grains of 

 silver visible to the naked eye, and indeed such as may be col- 

 lected by the fcrceps and extended under the hammer, may in 

 this way be extracted from the lead met with in commerce." 



Silica, fused into a glass with soda, detects the presence of 

 sulphur or sulphuric acid ; and oxide of copper is used to de- 



