Page:Proceedings of the Royal Society of London Vol 69.djvu/337

Rh 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the same alloy, each weighing about 5 grammes, were melted and slowly cooled, and at selected temperatures ingots were withdrawn from the furnace and chilled by immersion in cold water.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 If the alloy was partly liquid when withdrawn from the furnace, it generally became more or less granulated by the process of chilling, and the microscope then showed it to consist of large copper-rich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.