Frederick Guthrie on Eutexia. 465 



§ 201. So that we have the following results : — 



Eutectic alloy of Bismuth with 



Per cent. Q 



Lead contains 44*42 lead and melts at 122*7 



Tin „ 53*90 tin „ 133 



Cadmium „ 40*81 cadmium „ 144 



Zinc „ 7*15 zinc „ 248 



These weight-ratios are none of them simple atomic weight- 

 ratios. Of course, in many cases, metals can be fused together 

 in simple multiples of their atomic weights, but in most such 

 cases, on cooling the alloy, one of the metals will separate out 

 first and so destroy the ratio. In other cases an alloy richer 

 than the original alloy in regard to one of the metals, and 

 therefore a homologue of a subcryohydrate, separates, and 

 the original ratio is again disturbed. When we are dealing 

 with metals which, like antimony or arsenic, on the one hand, 

 are themselves halogenous, or with such strongly chemico- 

 positive metals as sodium on the other, we may and do get alloys 

 of atomic composition — metallo-metallic salts. But the tem- 

 peratures of fusion of these are never, as far as I am aware, 

 lower than that of either (both) of the constituents. They are 

 not eutectic. Nor is it easy to see any reason whatever why 

 they should be so. On the contrary, we should expect such 

 an alloy to be soluble in one or other of its elements, with de- 

 pression of temperature. The statement therefore that alloys 

 of minimum melting-points are got on mixing the metals in 

 certain simple ratios of their atomic weights, is presumably 

 to be put on one side. As the cryohydrate forms itself when a 

 salt-solution of any strength loses heat, so the eutectic alloy 

 forms itself when an alloy whose constituents are in any ratio 

 is cooled. The only exception to this is when the constituents 

 are in the ratio homologous with that between the consti- 

 tuents of the subcryohydrate. So-called "fusible alloys," or 

 "fusible metals/' are imperfect eutectic alloys. And it is 

 seen that the melting-points obtained by previous experi- 

 menters with bismuth-tin and bismuth-lead are considerably 

 higher than those above shown (see also § 20.6). 



§ 202. Further, if we take the eutectic alloy of bismuth-tin 

 and that of bismuth-lead, and treat them together as though 

 they were single metals, we get the eutectic alloy of the three 

 metals ; or either of these alloys with bismuth-cadmium gives 

 a corresponding tri-eutectic. 



These and certain other tri-eutectic alloys may form the 

 substance of a further communication. They stand in the 

 closest connexion with the cryohydrates of mixed salts, which 

 were discussed in §§ 109-116. 



