Specula * |Sf 
shades of green or yellow ; and if, when broken, the face 
Of the fracture exhibited a silvery whiteness, as bright 
and glistening as quicksilver, without any appearance of 
grain, or inequality of texture; then the degree of sa- 
turation of the metal, with the tin, was complete and per* 
feet. 
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, where broken, discovered a dull colour, with a coarse 
grain, like facetts ; the due saturation was exceeded, and 
there was an over proportion of tin in the metah 
These colours would be more distinct, if a small quan- 
tity of the metal were cast in a flask, which had been pre- 
viously smoaked, by a candle, made of resin mixed with 
tallow in which way 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 peculiar colour, so 9 
the heat of the melted mass, calcining some of the parti- 
cles on its surface, winch are in contact with the air 9 
these display the colour of the calces of those ingredients 
which prevail in the composition. Whence, it may be 
expected, that, if the copper is the redundant metal, the 
mass will exhibit a reddish tinge, which is appropriate to 
* This can always be done by degrees, and without any trem- 
ble, till the point of saturation is found ; whereas, if too much tin 
were added at first, there would be a necessity for melting inore 
copper separately, and repeating the whole process : and differ- 
ent specimens of copper will require different proportions of tin ; 
so that the due quantity can never be known, a priori , but on trial 
only. 
Vox. Ill, S 
