of Metals for reflecting c tielefcopes. 301 
which had given me fo much embarraffment and trou- 
ble during a courfe of near a hundred experiments, and 
in confequence thereof fell upon a method which ever 
after prevented it. 
I had hitherto always melted the copper firft, and 
when it was fufficiently fufed, I ufed to add the propor- 
tional quantity of tin ; and as foon as the two were mixed, 
and the fcoria taken off, the metal was poured into the 
moulds. I began to confider that putty was calcined tin, 
and ftrongly fufpedted, that the exceffive heat which the 
copper neceffarily undergoes before fuilon, was fufficient 
to reduce part of the tin to this ftate of calcination, which 
therefore might fly off from the compofition in th e form 
of putty, at the time the metal was poured into the flafks. 
Upon this idea, after I had furnifhed myfeif with 
fome more Swediih copper and grain-tin (both which I 
had always before ufed) I melted the copper, and having 
added the tin as ufual to it, caffc the whole into an in- 
got: this was, as I expedted, porous. I then melted it 
again, and as in this mixed ftate it did not acquire half 
the heat which was before neceffary to melt the copper 
alone, fo it was not fufficient to calcine the tin ; the fpe- 
culum was then perfectly clofe, and free from this fault ; 
nor did I ever after, in a fingle inftance, meet with the 
above mentioned imperfection. 
All 
