Oil the EffcB of a Mixture of Tin with Cold. 183 



and contribute to the certainty of their operations. I think this purpofe will be anfwercd, 

 by leaving them in pofTeflion of all the apprehenfion they have hitherto entertained refpeft- 

 ing the mixture of tin with gold. 



If it Ihould be thought that my experiments may not be as conclufive as at firft (ight 

 they appear, they will at lead produce no other effed than that of giving artifts fome ufe- 

 lefs trouble. They will with reafon attend to them, however fuperfluous they may be 

 fuppofed, and will prefer the care I recommend to the anxiety of working upon materials 

 rendered fufpicious by their harfhnefs, and fuppofed to be incapable of annealing on ac- 

 count of their containing tin. 



If it be allowable to form fome conje£tures on the frafture of plates of gold containing 

 tin, when fubjefted to the annealing heat, it may be prefumed, that the tin, which very 

 fpeedily melts, while the gold requires a very ftrong heat for its fufion — it may be pre- 

 fumed, I fay, that the parts of the tin intermixed in a fort of proportional equality with 

 thofe of the gold, tend to feparate by a fpeedy fufion and at a very gentle degree of heat; 

 that they remain without confidence between the parts of the gold, while the latter pre- 

 ferve the whole of their folidity, and do not lofe it even by the annealing heat ; whence it 

 feems, that the parts of the precious metal, when ignited among the coals, having no longer 

 the folid connedlion formed by the tin, but, on the contrary, having an infinite number of 

 foiall cavities occupied by particles of that metal in fufion, muft tend to difunion; whereas 

 the fame accident does not take place in the pieces which have refilled the annealing, and 

 have been laminated after cooling, becaufe the particles of tin have become folid by cooling, 

 and have recovered their original flate of union with the gold. 



This fra£ture of the compound does not take place with an alloy of gold and copper, for 

 an oppofite reafon to that which has here been explained ; namely, becaufe thefe two me- 

 fals require nearly the fame heat for their fufion. The efFe6t of annealing being therefore 

 equal upon both, the metals, notwithftanding this treatment, preferve their natural con^ 

 fiftence, even though the heat be carried near the point of fufion. 



In fupport of the opinion I have prefented, refpeding the fracture of thefe plates of gold, 

 I mull obferve that, by examining theit furfaces under the microfcope, a great number of 

 particles of tin may be diftinguiflied, which appear to be difengaged from the pores of the 

 gold; that thefe furfaces, being applied by an annealing heat to a plate of iron or filver, 

 adhere ftrongly by virtue of thefe particles of tin •, that they cannot be feparated but with 

 difficulty, and even tear up fome flight portions of the metal on which tliey were annealed 

 when they are fo feparated. 



I fball conclude thefe details, into which the fubje£l of my experiments has forced me to 

 enter, by remarking that the plates of gold here fpoken of may be kept entire, when annealed, 

 on a flat plate of metal, on which they may remain fupported till cold ; whereas, if placed 

 on ignited charcoal, they frequently bend or twill, and are fubjeft to break, particularly if 

 aot attempt is made to take them out with the tongs in their ignited ftatc. 



IX. On 



