The first of these two differences is, that the Acid being reduced into vapours when it acts on the Silver in cementation, its activity is thereby greatly increased: the second is, that in the crucible it sustains a vastly greater degree of heat than it can ever bear when it is in the form of a liquor. For, after it is once distilled and separated from its basis, it cannot sustain any extraordinary degree of heat without being volatilized and entirely dissipated: whereas, while it continues united with its basis, it is much more fixed, and cannot be separated but by a very intense heat. Consequently, if it meet with any body to dissolve, at the very instant of its separation from its basis, while it is actuated by a much fiercer heat than can ever be applied to it on any other occasion, it must operate upon that body with so much the more efficacy: and thus it comes to pass, that in cementation it has the power of dissolving Silver, which it would be incapable of touching if it were not so circumstanced. But herein Gold differs from Silver: for, whatever force the Nitrous or the Marine Acid may exert, when extricated from their bases in the cementing crucible, this metal obstinately refuses to yield to either of those Acids separately, and can never be dissolved by them, unless both be united together. Our cementation, therefore, is actually a parting process in the dry way. The Silver is dissolved, and the Gold remains unaltered. Nay, as the action of the Acids is much stronger when they are applied this way, than when they are used for dissolution in the moist way, the Nitrous Acid, which in the common parting process will not dissolve Silver unless its weight be double that of the Gold, is able in cementation to dissolve a very small quantity of Silver diffused through a large quantity of Gold. It sometimes happens, that after the operation the cement proves extremely hard, so that it is very troublesome to separate it entirely from the Gold. In this case it must be softened by moistening it with hot water. This hardness which the cement acquires is occasioned by the fusion of the Salts, which is the effect of too strong a heat. It was in order to prevent this, and that a due degree of heat might be applied, without the danger of melting the salts, that we directed the cement to be mixed with a considerable quantity of earthy matter incapable of fusion, such as brick-dust. A greater inconvenience will ensue, if the fire be made so strong as to melt the Gold: for then it will partly commix again with the other metalline substances dissolved by the cement, and consequently will not be purified.