3B DR. WALTER ROSENHAIN AND MR. SYDNEY I, ARCHBUTT ON THE 



this alloy and all those lying to the right of the point II Income perfectly 

 homogeneous. 



It will be seen in the diagram that the lines GH and KL have been continued as 

 dotted lines to the aluminium end of the diagram. This has been done because it 

 may be supposed that, if final equilibrium is attained in the freezing process, the 

 solid solution will react at the temperature of the line GH with the zinc which it 

 contains to form the compound Al 3 Zn 3 , and this in turn will undergo decomposition 

 when the temperature of the line KL is reached. This supposition, however, involves 

 the assumption that the y solid solution contains the zinc as " free" zinc, while in the 

 S solid solution the zinc is present in the "combined" state, i.e., in the form of 

 molecules of the compound Al 2 Zn 3 . Just to the right of the line GFK of the 

 diagram there can be no doubt that we have in the $ meteral a solid solution in 

 which the compound /8 is the solvent and free aluminium is the solute ; this solid 

 solution, however, is merely one end of a continuous series whose opposite extreme is 

 represented by a solid solution in which aluminium is undoubtedly the solvent and 

 the compound the dissolved body. The question arises whether the phenomena of 

 dissociation or ionisation which are known to occur in dilute aqueous solutions also 

 occur in these solid metallic solutions. Where the solution is a strong one, i.e., in 

 the present case for alloys containing more than 35 per cent, of zinc, there is ample 

 evidence that the compound ft preserves its identity in the solid solution, but for 

 lower concentrations there are two reasons why experimental evidence is not 

 available. 



In the first place, for all alloys lying to the right of the point H in the diagram, 

 the reaction between the two constituents of the y solid solution must take place in 

 entirely solid metal. Such reactions are not unknown, but in most cases they are 

 very slow, and in the present instance the reaction in question is known to be slow 

 even when one of the reacting phases is a liquid. It is, therefore, not surprising to 

 find that even at the slowest rates of cooling which are possible for the observation of 

 < "ling-curves the reaction in these alloys takes place too slowly to be indicated by a 

 heat evolution. The absence of arrest-points along the line GH beyond the point H 

 is, therefore, no definite evidence that the reaction does not take place gradually if 

 tun.- !* jrivim at a favourable temperature. Further, microscopic evidence is difficult 

 to obtain since microscopically the y and S solid solutions are both homogeneous 

 bodies. The only test would be that of tracing the effects of the decomposition at 

 the temperature of the line KL by looking for the appearance of the a phase in the 

 annealed and slowly-cooled alloys. An endeavour was made to do this, and the 

 think it probable that the a phase can be traced up to a concentration of 

 aluminium us high as 80 per cent., but it must be admitted that this method is not 



ry reliable iii this instance. The reason lies in the fact that even the homogeneous 



this series never present a perfectly clean polygonal structure the surfaces 



of the crystals always show a certain amount of marking or pattern, while minute 



