WO DR. WALTER ROSENHAIN AND MR. SYDNEY L. ARCHBUTT ON Till. 



the alloys. Some experiments have been made by taking cooling-curves of a few 

 ternary alloys of aluminium, zinc, and iron with a view to establishing the connection 

 between the small heat evolutions under discussion and the presence of the compound 

 referred to. It would seem, however, that the iron-aluminium diagram itself requires 

 to be worked out afresh and that the relationships between these three metals are of 

 a complex kind, so that a great deal of further experimental work will be necessary 

 before the truth of the suggested explanation can be tested. In view of these 

 circumstances the authors have not thought it desirable to delay the publication of 

 the present research until this further and larger investigation could be completed. 

 They therefore content themselves with indicating this series of heat evolutions on 

 the diagram, but without drawing a line through them or offering more than the 

 tentative explanation outlined above. 



We now pass on to the consideration of the micro-structures of the alloys. As has 

 already been indicated, much of the evidence for the lines of the diagram, and 

 particularly for their interpretation, is based upon microscopic evidence, and the 

 micro-structures will be described from this point of view. 



For the purpose of microscopic examination small specimens were, as a rule, cut 

 from the ingots used for cooling-curve determinations. In some cases complete 

 vertical sections of the ingots were examined in order to avoid risks of error from 

 segregation. The polishing of the specimens near both ends of the series proved very 

 difficult. Most of the specimens were etched with a 10-per-cent. solution of caustic 

 potash in one or two cases weak nitric acid was used. 



Commencing at the zinc end of the diagram we have first a very narrow area 

 marked in fig. 4 as representing alloys consisting entirely of the meteral a. This 

 designation includes pure zinc and the solid solution of aluminium in zinc. The exact 

 limits of the solubility of aluminium in solid zinc have not been finally determined, 

 although an alloy containing 1 per cent, of aluminium has been found to show signs 

 of the presence of eutectic after 24 hours of annealing at a temperature just below 

 the freezing-point of the eutectic. This observation indicates that unless the rate of 

 solution is extremely slow the limit of the a region lies within 1 per cent, of pure 

 The line indicating the limit of this region is therefore shown as a dotted line 

 only, and has, for convenience, been drawn further to the right than the evidence 

 justifies. 



The diagram next indicates a group of alloys consisting of the meteral a embedded 



in eutectic, the constituents of the eutectic being termed a and ft above the line IJ, 



and a. and y below that line. The general structure and appearance of one of these 



s shown in Plate 5, fig. 8, under a magnification of 150 diameters. This 



the alloy containing 98 per cent, of zinc slowly cooled, and shows a with 



ies of eutectic. If such a specimen is quenched from a temperature 



256 C.) the appearance of the micro-structure under moderate magni- 



tion, such as that of fig. 8, is as shown in that figure, but under higher 



