410 



Messrs. C. T. Heycock and E. H. Neville. [Feb. 26, 



It occurred to us, and this is the method of the paper, that we could 

 simplify the phenomena by a systematic chilling of the ingots at 

 selected temperatures. A number of small ingots of the same alloy 

 were placed in separate tubes in a bath of tin, together with a recording 

 pyrometer, the temperature was raised above the freezing point of the 

 alloy, and the whole very slowly cooled, the slow cooling being an 

 essential feature of the experiment. Ingots were then extracted at 

 selected temperatures and rapidly chilled by immersion in water. 

 The microscopic examination of these chilled ingots showed that it 

 was quite easy to distinguish the large crystals, that had formed during 

 the slow cooling preceding the chill, from the matter that was liquid 

 when the ingot was withdrawn from the furnace. 



Successive chills of an alloy exhibit the solid growing in amount as 

 the temperature falls, and finally show the ingot completely full of solid. 

 We thus obtain, with very reasonable accuracy, the temperature of com- 

 plete solidification of the alloy ; and by applying the method to alloys 

 with different percentages of tin we have traced a new curve, the 

 " solidus," or curve of complete solidification. The solidus of the 

 bronzes is the second line of the diagram. It is a remarkable line, 

 made up of sloping, horizontal and vertical branches. As in the 

 freezing-point curve, each branch corresponds to the crystallisation of 

 a different solid. 



In the notation of Professor Eoozeboom, the upper curve ABC ... I, 

 is called the liquidus, because all wholly liquid alloys lie above it ; 

 and the solidus, A&7cc/e/E2E 3 H'H"I, is so named because all wholly 

 solid alloys lie below it. 



The solidus of the bronzes is remarkable for the very narrow range 

 of temperature within which some alloys pass from the wholly liquid 

 to the wholly solid state. 



According to Eoozeboom's theory, each sloping branch of the solidus, 

 and there are four such in the diagram, corresponds to the crystalli- 

 sation out of the liquid of a different series of solid solutions, each 

 vertical part to the crystallisation of a pure body, and each horizontal 

 part to the case of the solid alloy at temperatures immediately below 

 the solidus, being a complex of two substances. Our examination of 

 the chilled ingots has completely verified all these statements. 



The evolutions of heat observed by Eoberts- Austen and Stansfield at 

 temperatures corresponding to the point C, D, G and H are due to 

 definite chemical transformations in which one solid is decomposed 

 and another is formed. Chills taken immediately above and below 

 these critical temperatures reveal the nature of each change most clearly. 



The transformations at C, D, and especially at H, are very slow and 

 do not become complete unless the temperature is maintained constant 

 for hours or days at a point slightly below the transformation tem- 

 perature, but all these changes can be made to agree exactly with 



