On Surfusion in Metals and Alloys. 453 



is stirred, the first point to be recorded will be the upper or tin point 

 (185), followed by the eutectic point (183). If the "alloy surfuses, 

 the first point recorded will be the lowest point (176), followed in 

 turn by the eutectic. In general, therefore, only two freezing points 

 can be obtained in a single record. 



In the case of salts the crossing of the curves of solubility has 

 already been observed by H. le Chatelier and by Dahms, but in the 

 case of alloys, experimental evidence has hitherto been wanting. 

 The silver-copper series presents many analogies to the lead-tin 

 series. Heycock and Neville*, in their excellent work on the com- 

 plete freezing point curves of many series of alloys, have calculated 

 what the ideal freezing point curve of the silver-copper series would 

 be, but the present paper affords, I believe, the first experimental 

 evidence as to the identity of the behaviour of saline solutions and 

 metallic alloys as regards selective surfusion. 



Prolonged experience in these methods of manipulation may prove 

 that it is possible to effect the separation of a particular metal or 

 definite groups of metals by dropping in (during the surf usion of the 

 fluid mass) a fragment of the same metal or of the particular group 

 of the associated metals it is intended to separate. It is well known 

 that the introduction of a fragment of the same metal or of an iso- 

 morphous metal or alloy will determine its solidification. Such a 

 method may readily be employed in studying the surfusion of salts. 

 In the case of metals, so far as my own experiments go, the sur- 

 fused state is singularly unstable, for it may be disturbed even by 

 very slight tremors. It remains to be seen whether it is possible to 

 arrange the experiments in such a way as to maintain metals and 

 alloys for an indefinite time in Ostwald's meta-stable condition 

 which would need the presence of a particle of solid matter to 

 induce the solidification of the mass. 



Ostwald applies to the change from liquid to solid the equation 

 which represents the gas-liquid change. In the former case there 

 are, however, the three phases, solid, liquid and gas, present, and a 

 complete expression of the change must take account of all three. 

 Thus during eurfusion the gas phase is in equilibrium with the liquid 

 phase, but when solidification has begun the gas phase must also be 

 in equilibrium with the solid phase. It is possible that the solidi- 

 fication of the mass may be started by crystals deposited directly 

 from its vapour so that solidification of a surfused metal may be 

 started by crystals from its own metallic atmosphere. The amount 

 of gas evolved by a solid metal is, of course, very small. Evidence 

 of the vaporization of metals at very moderate temperatures is not 

 wanting. Dema^yf showed in 1882 that in vacuo metals evaporate 



* 'Phil. Trans.,' A, vol. 189 (1897), p. 25. 

 t ' Comptes Rendus,' 1882, vol. 95, p. 183. 



