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gredients 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 ap- 

 propriate to the calx of copper; and, if the tin be preva- 

 lent, a blueish die ought to appear. Either of these co- 

 lours, therefore, appearing unmixed, shews the redundance 

 of that metal, to which each belongs. And, as brass, 

 when cast alone, has always a yellow tinge, so, when 

 these three colours are exhibited in a cloud-like mixture, 

 they shew an equality and due proportion of their re- 

 spective metals in the composition. When too large a 

 mass of the metal is cast together, its intense and lasting 

 heat calcines the surface so deeply, as (when exposed to 

 the air) to obscure the colours; so that a small quantity 

 Avill best serve to exhibit them. 



As to the method of casting the mirrors, it has been 

 directed, to leave the ingate, or superfluous part of the 

 cast, so large, as to contain a quantity of metal, equal 

 to that in the mirror itself; Avhich would occasion a great 

 waste of it, and render it not easy to cast, at once, more 

 than one mirror in each mould; and even this might be 

 done so injudiciously, as not to afford security against 

 a miscarriage of the cast. But it will appear, that this 

 great quantity of metal, and incommodious manner of 

 casting it, are by no means necessary. However, a judg- 

 ment cannot be formed, of what may be the safest and 

 most eligible method for casting the mirrors, unless it 

 be considered, what are the circumstances attending this 



operation, 



