CONSTITI'TION OF Till-: AI.I.oYS OK Al.rMIMl'M AM- /INC. 329 



evolution slightly, but its principal effect is the gradual depnttioa of the temperature 

 at which tin- change takes place. 



It was thought at first that a curve joining these points near the aluminium end 

 might represent tin- solidus of the alloys, while the points near the other end of the 

 series were suspected of tunning another horizontal line running into the liquidus and 

 indicating the presence of another alumininin-/.inc compound. These heat evolutions, 

 however, although very minute are very definite, and the temperatures at which they 

 occur are so well defined that tin- experimental evidence, is sufficient to negative the 

 supposed existence of a hori/.ontal line in this case. The certainty with which these 

 minute peaks recur on repeated cooling-curves of the same alloy has already been 

 illustrated in fig. 3, where three successive cooling-curves of the alloy No. 35 are 

 plotted side by side. The supposition that the upper part of the curve might 

 represent the solidus was also negatived by the indications of quenching experiments 

 which, by demonstrating the absence of an upward step in the solidus between H 

 and D, further negative the supposed existence of a compound whose formation 

 might lie represented by these am 



The explanation which at first sight suggested itself for this Hue of arrest-points is 

 that they are due to an allotropic change in aluminium, or in the phase rich in 

 aluminium, and that this change occurs at lower temperatures with increasing 

 concentration of zinc. If the line of the points in question lay entirely below the 

 solidus, this explanation might be regarded as feasible, but in the alloys from about 

 3 per cent, to 66 per cent, of zinc, these points occur very definitely a)>ove the solidus 

 and a transformation or inversion of any kind in a solid phase which is surrounded by 

 its mother liquor can only occur at a constant temperature since the composition of 

 such a solid phase at a given temperature is the same for the whole series of alloys in 

 which it occurs, and the same inversion must occur in all such Ixxlies at the same 

 temperature. Meta-stable conditions cannot be invoked to explain away this 

 difficulty, since the meta-stable formation of "cores" in such alloys implies the 

 presence of a solid containing less zinc than the average composition of the alloy, and 

 since the line of points in question slopes upwards towards the aluminium end of the 

 diagram these meta-stable cores would undergo transformation or inversion at a 

 hnjlii-r temperature than that which would occur under equilibrium conditions in the 

 same alloys. These considerations, therefore, afford no explanation for the depression 

 of these points with increasing concentration of zinc. 



The oidy explanation which the authors feel justified in suggesting is that the 

 occurrence of these points, or rather their continual depression with increasing zinc 

 content, may be due to the action of a third component substance which enters into 

 the alloys as an impurity. Such an impurity is found in the form of 0'20 per cent, of 

 iron present in the aluminium employed, while a certain amount of silicon is also 

 presrnt in the alloys. The author's suspicion has fallen principally upon iron since a 

 few isolated crystals of an iron aluminium compound have been observed in most of 



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