1892.] Specimens of Rock exposed to High Temperatures. 395 



the corresponding results obtained with lead-zinc-silver alloys shown 

 in fig. 8, these lines clearly indicate the effects due to the formation 

 of AgZn 5 and Ag 4 Zn 5 ; the formation of the first leading to a marked 

 bulge inwards on both sides of the curve at the lowest part ; and that 

 of the second to an outward bulge somewhat higher up in the left- 

 hand branch, as compared with the dotted line ab connecting the 

 upper part of the curve with the point & on the base line, representing 

 the alloy of bismuth and zinc formed in the absence of " solvent " 

 metal. 



The points marked A, B, and C represent certain mixtures ex- 

 amined that did not separate into two different alloys, i.e., that formed 

 "real" alloys at 750. 



Leaving out of sight the lower bulges inwards and outwards, due 

 to formation of AgZn 5 and Ag 4 Zn 5 , it is evident that the critical curve 

 for bismuth-zinc-silver lies inside that for lead-zinc-silver, just as that 

 for bismuth-zinc-tin lies inside that for lead-zinc-tin. Further, the 

 curve for bismuth-zinc-silver lies outside that for bismuth-zinc-tin 

 just as the curve for lead-zinc-silver lies outside that for lead-zinc-tin. 

 With silver as " solvent " metal the limiting point lies to the left of 

 the central line of the triangle (bismuth or lead side) ; whilst with 

 tin as solvent metal it lies to the right (zinc side). In the first case 

 the ties uniformly slope downwards to the left, and in the second, to the 

 right so far as the upper ones are concerned ; although a slope to the 

 left is observed with the lower ones for the lead-zinc-tin alloys, sug- 

 gesting the existence of a tendency to form a definite compound of 

 lead and tin (possibly SnPb 3 ), no analogous tendency being notice- 

 able with bismuth and tin. 



The author has much pleasure in acknowledging the assistance 

 afforded him by Mr. Sydney Joyce in carrying out the further experi- 

 ments above described. 



IV. "Note on some Specimens of Rock which have been 

 exposed to High Temperatures." By Professor T. G. 

 BONNEY, D.Sc., LL.D., F.R.S. Received December 18, 

 1891. 



The effects of raising several varieties of rock to temperatures 

 which, though high, in some cases were below those required to pro- 

 duce fusion were described to the Royal Society by Mr. Rutley in 

 1886.* The following notes may be useful as a continuation of a 

 .subject which is not without interest owing to its bearing on natural 

 processes. 



Some time since, Mr. J. Postlethwaite, F.G.S., of Keswick, kindly 

 * Hoy. Soc. Proc.,' rol. 40, p. 430. 



