112 MESSRS. WALTER ROSENHAIN AND P. A. TUCKER. 



solution at lower temperatures if the alloy is slowly cooled, and this fact has 

 considerably complicated the determination of the limiting solubility. The fact that 

 very gradual cooling would be required to produce this complete equilibrium is borne 

 out by microscopic evidence, as well as by the cooling-curves described above. 

 Figs. 23 and 24 (Plate 6) are photo -micrographs, at a magnification of 150 diameters, 

 of alloys containing 10 and 15 per cent, of tin respectively, cooled from fusion in 

 an ordinary laboratory furnace. Considerable quantities of eutectic appear in both, 

 but, as has already been indicated, these alloys gradually become homogeneous on 

 exposure to a temperature of 175 C. 



In the alloys lying Ixjtween 8 and approximately GO per cent, of tin the cooling- 

 curves show the existence of changes involving an evolution of heat at a temperature 

 which lies at 149 C. for alloys above 18 per cent, of tin, and at lower temperatures 

 for alloys of lower tin content. In order to correlate these evolutions of heat with 

 changes in the micro-structure of the alloys it was necessary to cool specimens of the 

 alloys from a temperature just above the recalescence point at so rapid a rate as to 

 more or less suppress the change in question. It was evident that from a temperature 

 so low as 149 C. mere quenching in cold water would not be sufficient for this 

 purpose, and quenching in liquid air was therefore employed. 



For this purpose a Dewar vessel containing a considerable quantity of liquid air 

 was brought to the door of a small electric oven in which the specimens had been 

 heated ; the quenching operation was carried out by removing from the oven a small 

 slab of uralite on which the specimens had been placed for this purpose and allowing 

 the specimens to slide quickly and directly into the liquid air. The specimens were 

 made of small dimensions, and the violent ebullition of the liquid air which they at 

 first produced subsided very rapidly. As soon as the specimens had become quiescent 

 in the bath they were removed from the liquid air and allowed to lie on the table 

 until they had regained the ordinary temperature, when they were prepared for 

 microscopic examination, as previously described. The object of using liquid air in 

 this instance, it should be noted, was simply to ensure the most rapid possible rate 

 of cooling, and not in any way to test the effect of liquid-air temperatures upon the 

 alloys. It was, however, necessary to ascertain whether the mere fact of exposure to 

 such a low temperature produced any change in the micro-structure of the alloys, and 

 to test this point duplicate specimens of the alloys were immersed in liquid air for a 

 similar length of time, but the immersion in this case only took place after the 

 temperature of transformation had been passed ; specimens of pure lead and of pure 

 tin were included in this experiment. A third set of similar specimens was then 

 allowed to cool very slowly in the electric oven itself, and the micro-structure of the 

 three sets was subsequently compared. In the case of pure tin, the experiment was 

 tried in view of the fact that tin is known to exist at low temperatures in the allo- 

 tropic form of a grey powder, and it was supposed that some sign of this phase might 

 be detected in the specimen cooled in liquid air ; this was not the case, however, and 



