Cooling curves show that this change is exothermic and abrupt, much heat being evolved by the alloys near /, but little by alloys near G. Herein the transformation at G is the reverse of the trans- formation at C. The change from the rounded y crystals to the plates of E' with liberation of liquid is perfectly shown by our photographs of alloys chilled near the G temperature.
When an FG alloy has cooled below G the phase E' crystallises out of a liquid which is becoming richer and richer in tin. This process goes on between the G temperature of 633 and the H temperature of 400. Below 400 the FG alloys follow the same course as the next group.
8. The GH Alloys, containing from 42 to about 87'5 atomic per cents, of Tin. When these alloys begin to crystallise they deposit plates of E', and this process goes on until the liquid has the composition H and the temperature is 400. At this temperature the body E' becomes less stable than the body H, and the reaction E' + liquid = H, com- mences. This reaction ought to complete itself isothermally until either all the E' or all the liquid is transformed ; but in our experi- ments we find that the reaction is soon arrested through the plates of E' becoming completely coated with H, and thus protected against further action by the liquid. When this has occurred, the temperature begins to fall again, and H crystallises out of the liquid until the eutectic point I is reached.
The result of this imperfect reaction is that we have four compart- ments in the space E 3 SUH, in each of which one of the three phases that exist would be absent if the equilibrium transformations had been completed. This part of the figure assumes that the formula of the H-body is CuSn. If this should prove erroneous the line H'T would have to be shifted, but otherwise the diagram would be unchanged. In the four compartments of the area EsSUH, Professor Roozeboom has suggested the excellent plan of placing a bracket round the symbol of the phase that has no right to be present
9. The HI Alloys, containing from 87 '5 to 98'3 atomic per cents, of Tin. In these alloys the solid first forming is H, and the diagram suffi- ciently explains itself.
10. The IK Alloys, with more than 98 - 3 atomic per cents, of Tin. These alloys contain combs of pure tin in a eutectic of H + Sn.
Some Observations and Qualifications.
The Solidas. We have drawn this everywhere as a definite line, but our method of determining it, by the inspection of chilled alloys, is probably not so exact as the method by which the liquidus has been determined, and therefore further work on the solidus is desirable. We think that a determination of the melting point as distinguished from the freezing point would be a promising plan.
