492 Compojition and FaHrication of the 



the metal much more compaft and folid, and indeed much more heautiful, as one may 

 experience by comparing the compofition tuith arfenic, with the fame compofitlon of copper 

 and tin tukhout arfenic. In general I find one * ounce of arfenic fufficient for one pound 

 of the metal. A much greater quantity of arfenic may be ufed, without any difadvantage 

 to the beauty or compaftnefs of the metal ; but then it is too apt to tarnifti if expofed to 

 the air for fome time : three quarters of an ounce, or an ounce of arfenic, to one pound of 

 compofition, ^will not tarnifli in the lead degree. Indeed, the reafon why the metals gene- 

 rally made ufe of for fpecula, tarniih when they are much expofed to the air, is becaufe the 

 quantity of copper in their compofition is not nearly faturated, and the acid contained in 

 the air, by a£ling upon it, extrafts the copper from the tin, and turns the metal into a 

 dirty or dingy colored fpeculum, and which (befides the great lofs of light) caufes the com- 

 mon refledling telefcopes to fhew all obje£ls of a dirty red, or yellowifli colour. This, 

 however, is not the cafe in the metals made of the above compofition ; for as the copper is 

 compleatly faturated, the air cannot a£t upon it in the leaft degree. I muft not, however, 

 pafs over one caution in the mode or manner of melting the compofition ; and that is, that 

 the copper mull be melted firfl: of all, and rendered as fluid as poflible, then the brafs and 

 filvcr muft be added, and the whole fluxed with the common black flux, made of two parts 

 of tartar to one of nitre, or by ftirring the melted mixture with a wooden fpatula of birch, 

 and made as fluid as polTible. The tin muft now be added, and the whole poured off im- 

 mediately, after it is once ftirred together; for if the mixture is continued on the fire fome 

 time after the tin is added to it, it will always prove porous afterwards, though it be melted 

 a fecond time with the fmalleft heat poITible. As I ever found this to be the cafe, I.na- 

 turally conje£l:ured, that the metal would be moft folid and free from pores, when the tin 

 remained the leaft time poflible in a ftate of calcination. Experience determined the truth 

 of my conje£lure, and I now find, that the beft method poflible to make this compofition 

 to the greateft advantage, is to melt the copper as fluid as poflUble, and flux it with the 

 black flux ; to melt the tin in a feparate crucible by itfelf ; to take the two crucibles out of 

 the fire, and pour the melted tin into the fluid copper ^ and ftir it inftantly with a wooden 

 fpatula, and pour the whole immediately into a large quantity of cold water. The fudden 

 chill from the cold water divides the melted mafs into an infinite number of fmall particles, 

 and by that means cools it inftantaneoufly, and confequently prevents the tin from cal- 

 cining fenfibly ; and hence I have always found that in the fecond melting, the compofition 

 was entirely free from pores, even though no arfenic had been employed. Yet the addition 

 of arfenic ever rendered it much more compaft, and indeed t fpecificaliy heavier, as well as 

 more brilliant and beautiful. On reverfing the procefs, if the tin is put into the bottom 



* One ounce of arfenic will, however, fufficiently aft upon and iind three pounds weight of the metal, 

 fo that it ihall never tarniOi by the air. 



t The fpecific gravity of the compofition Itielf is J,78, with the addition of one ounce of .vfenic to one 

 pound of metal, is 8,89. 



of 



