125 



pearance of grain, or inequality of texture; then the de- 

 gree of saturation of the metal, with the tin, was com- 

 plete and perfect. 



Secondly. If the surface of the metal became of a 

 -dun or mouse colour, and especially if of a brown or 

 red; and, when broken, the fracture exhibited a more 

 yellow, or tawny hue, than that of quicksilver; then 

 the quantity of tin in the composition was deficient, 

 and it was necessary to add more.* 



■ Thirdly. If the colour was an uniform dull, blue, like 

 lead, and,' where broken, discovered a dull colour, with 

 a coarse grain, like facetts; the due saturation was ex- 

 ceeded, and there was an over proportion of tin in the 

 metal. 



These colours would be more distinct, if a small 

 quantity of the metal were cast in a flask, which had been 

 previously smoaked, by a candle, made of resin mixed 

 with tallow ; in which Avay I used to prepare the moulds. 

 I attribute the formation of the colours to this: that, ,as 

 the calx of every metal has its own pecuUar colour, so, 

 the heat of the melted mass, calcining some of the 

 particles on its surface, which are in contact with , the 

 air, these display the colour of the calces of those in- 



voi. X. R . . gredients, 



. '''* This can always be done by degrees, and without any trouble, till 

 the point of saturation is found; whereas, if too much tin were added at 

 first, there would be a necessity for melting more copper separately, and 

 repeating the wljole proceBs: and different specimens of cppper will require 

 different proportions of tin; so that the due quantity can never be knowp, 

 a priori, but on trial only. 



