On the Constitution of Copper -Tin Alloys. 321 



No one dealing with a subject which has been so much studied can 

 give a comprehensive view of the phenomena without largely utilising 

 the results of previous workers. For example, in the pyrometric work 

 needed to determine the freezing-point curve ACDGH of our diagram 

 we have been helped in some important particulars, such as the singu- 

 larities at G and H, by the results already obtained by Sir W. Roberts- 

 Austen and Dr. Stansfield.* This is still more the case with regard 

 to the evolutions of heat which take place in the solid alloys as they 

 cool, and which are important indications of internal change. The 

 eutectic line b' C', and the very valuable curve XD' E' F', are also due 

 to them. We have, however, re-examined and verified by experiment 

 almost the whole of these pyrometric data. 



The microscopic examination of the unchilhd alloys is also not new, 

 interesting results having been obtained by Charpy and by Stead ; for 

 example, the existence of the compounds CusSn and CuSn has been 

 strongly suspected, if not quite proved. But it is our opinion that 

 conclusive results will never be obtained except by the examination 

 of alloys systematically chilled at selected temperatures, and it is in 

 this method, and in the interpretation of results derived from it, that 

 the new part of the work is to be found. Moreover, even with the 

 help of the chilled alloys, the interpretation would have been very im- 

 perfect without the aid of Professor Bakhuis Roozeboom's Theory of 

 Solid Solutions. We are also indebted to that gentleman for many 

 valuable suggestions directly concerning these alloys, and for kind 

 encouragement. 



The method and purpose of chilling the alloys can be described in a 

 few words. A number of small ingots of the same alloy, each weighing 

 about 5 grammes, were melted and slowly cooled, and at selected 

 temperatures ingots were withdrawn from the furnace and chilled by 

 immersion in cold water. We thus, to a very large extent, stereotype 

 the structure existing in the alloy at the selected temperature, and we 

 can examine it in the usual way by polishing and etching. If the 

 alloy was partly liquid when withdrawn from the furnace, it generally 

 became more or less granulated by the process of chilling, and the 

 microscope then showed it to consist of large copper-rich crystals 

 formed before the chill and surrounded by a network of much smaller, 

 but very similar, crystals formed during the chill, the whole being 

 embedded in tin-rich mother substance. We can thus determine by 

 the microscope how near to complete solidification the alloy was at the 

 moment of chilling. It was in this way that we traced the branches 

 Ab and Icdef of the solidus. In the above case it is hardly correct to say 

 that the structure existing before the chill is stereotyped by it, but 

 it is sufficiently recorded. On the other hand, when an alloy has 

 solidified before the moment of chilling, further changes are either 



* ' Fourth Report on Alloys,' February, 1897. 



