Page:Proceedings of the Royal Society of London Vol 69.djvu/341

Rh another break in the series of mixed crystals, but we have not been able to detect a corresponding break in the solidus. However, we propose to call the mixed crystals of the branch cdef y mixed crystals to distinguish them from the ft crystals of the branch k, and at Pro- fessor Roozeboom's suggestion we have drawn a sloping line from c to record the possible gap in the series between the last of the ft crystals and the first of the 7.

Thus the branch ABLC of the liquidus deposits a mixed crystals, the branch CD deposits ft mixed crystals, and the branch DEFG deposits 7 mixed crystals.

The branch 'GH of the liquidus deposits crystalline plates of the substance E', which is nearly or quite pure Cu 3 Sn.

The branch HI deposits crystals of the substance H, which we think is probably CuSn, although it may have an impurity in solid solution.

The liquid of the branch IK deposits crystals of pure tin.

Below the lines Ablcdef, E 3 H', and H"K' the alloys should be wholly solid, but on -account of imperfect transformations this is not in practice everywhere the case.

The points B, L, C, D, E, F, G, H, I divide the alloys into groups having special qualities. We shall now trace the complete cooling of an alloy from each group.

1. The AB Alloys, containing less than 5 atomic per cents, of Tin. When these alloys begin to solidify they form, while in the area AftC, a mixture of a crystals and liquid. When the temperature of an alloy has fallen below the line Ab, it consists of uniform a crystals embedded in a very little tin-rich mother substance, which owes its existence to imperfect adjustment of the equilibrium between solid and liquid during the last stages of solidification. This small residue of mother substance is absorbed at temperatures below 1C by the a crystals, and modifies their margins, but, in spite of some deceptive appearances, these alloys, at all temperatures below Ab, consist sub- stantially of one phase, namely, the uniform solid solution a.

2. The BL Alloys, containing from 5 to 1 3 atomicper cents, of Tin These alloys commence to solidfy by the' formation of a crystals, but when the temperature C is reached the reaction

a crystals + C liquid = /3 crystals

commences and continues until the C liquid is all exhausted. Thus, when the temperature begins to fall below C, the alloy is wholly solid and consists of a crystals of the percentage b embedded in ft crystals of the I percentage. These ft crystals are uniform and play the part of a mother substance to the a crystals, which now grow at the expense of the ft, while the solid alloy cools through the range of temperature W. This growth in the solid is very remarkable in alloys near the