4G 
MESSRS. C. T. HEYCOCK AND E. H. NEVILLE ON 
lower temperatures suffers a change into what we must suppose is y. We give 
(fig. 41) a photograph of a v.s.c. chill of Sn 17, chilled at 745°, which, as it contains 
no chill primaries, may be taken to indicate the maximum amount of /3 that can exist 
in this alloy. The chills of Sn 18, Sn 19, and even Sn 19’5, at this temperature are 
exactly similar to that given of Sn 17, except that they contain less We also give 
a photograph (fig. 42) of Sn 17, chilled at 731°, as it emphasises the symmetry of the 
combs and their great resemblance to a in form. The ingot was not es^^ecially 
slowly cooled before the chill, so that the detail is small. The ingot must have cooled 
too rapidly through the D temperature for the ^ combs to suffer much change, though 
a higher power makes it evident that their margins are softened by a commencement 
of solution. Other experiments show that if the ingot had not been chilled, the 
/3 combs would have disappeared at lower temperatures. The transformation at the 
D temperature is a slow one, though it probably goes on more rapidly a few^ degrees 
below that point, so that it is possible to find /3 combs, more or less recognisable, in 
alloys somewhat slowly cooled and chilled a little below D. Sn 18 chilled at 735° 
shows the partial solution of the which is recognisable in darker irregular patches. 
These retain, in a distorted form, some semblance of the shape of the ^ combs. This 
ingot was slowly cooled through the D temperature, hut enough time was not allowed 
for the completion of the change. We do not give a photograph of this ingot. The 
same process of solution of the ^ is seen in the photograph (fig. 43) of Sn 18 chilled 
at 734°. Here the ^ combs must have been originally very symmetrical, but as the 
ingot was not slowly cooled liefore the chill, they are easily recognisable in spite of 
the lower chilling temperature. We have ingots of Sn 17 and Sn 19 which show the 
same partial solution of the One of these, a chill of Sn 19 at 733°, is reproduced 
in fig. 44. 
When a patch of /3 undergoing this change is examined with a higher power, one 
sees that the grain of the material, originally fine and uniform, becomes coarser and 
develops into a flaky pattern of grey and white, and sometimes spherulitic crystals 
of a tin-rich material develop in the heart of what were uniform /3 crystals. All these 
obscure changes, however, result in the complete disappearance of the and the 
formation of a very uniform material, provided the alloy is cooled slowly to the 
solidus cd and then chilled. We give a photograph (fig. 45) of Sn 18 very slowly 
cooled to 728° and then chilled, in which the disappearance of the ^ is practically 
complete. The fine straight polygonal boundaries are present, but they have little or 
nothing to do with the original jd. 
This apparent uniformity in the Cl) alloys, when chilled below the solidus, has been 
confirmed in instances too numerous to mention in detail. For example, a very slow- 
cooled chill of Sn 18 at 728° was uniform, also one at 650°, and that at 550°, of which 
we give a photograph (fig. 48), is very neaily so. Again, very slow-cooled chills of 
Sn 19 at 719° and 710° are uniform except for polygonal boundaries; and the chills 
at 680° and 629° show, even after repeated ignitions to prange, no pattern except a 
