324 Messrs. C. T. Heycock and F. H. Neville. 



(8) There is also the substance D', which is never found in contact 

 with the liquid. The substance D' is pure at the point D', where we 

 believe it to be the compound Cu 4 Sn. This phase will be considered 

 somewhat later. 



The relations of the first seven phases can best be stated by con- 

 sidering the solidus (or, as we are inclined to call it, the melting- 

 point curve). The solidus is a broken line consisting of the branches 

 Ab, Icdef, E 2 E 3 , and H'H". 



The solidus and the liquidus (or freezing-point curve) are so related 

 that if we draw a horizontal, that is, an isothermal line, cutting the 

 solidus and liquidus, the points of intersection give the percentage 

 compositions of the solid and liquid that can exist in equilibrium at 

 the given temperature. To take an example, the isothermal at 900 C. 

 cuts the lines Ab and ABLC in points which correspond respectively 

 to a uniform solid containing 3 atomic per cents, of tin, and a 

 liquid containing 11 atomic per cents. These two would be in 

 equilibrium, for when the liquid was cooled it would begin to deposit 

 the solid, and when the solid was heated it would begin to melt and 

 form the liquid. 



Whenever a branch of the solidus is sloping, as Ab, or curved, as 

 Icdef, the solid phase is one of a series of mixed crystals. On the 

 other hand, when a branch of the solidus is vertical, as we have drawn 

 EoEs and H'H", one can infer that mixed crystals are not formed. It 

 is possible that we are wrong in drawing E^Es and H'H" quite 

 vertical ; the phase E' may here consist of Cu 3 Sn having some H in 

 solid solution, and the phase H may also contain some Cu 3 Sn or tin in 

 solid solution, in which cases the solidus would not be a vertical 

 straight line. But we have several reasons, some of which will be 

 stated later, for thinking that the mutual solubility of these bodies is 

 not great. 



The angle C of the liquidus indicates that the composition of the 

 solid phase changes abruptly at this temperature, for while the branch 

 ABC corresponds to the solidus Ab, the branch CD corresponds to the 

 solidus Ic. The angle C was a great stumbling-block to us so long as 

 we only examined alloys that had not been chilled, but Roozeboom's 

 theory explains in the most perfect manner all the phenomena at this 

 angle. It tells us that just above the temperature C the cooling 

 saturated liquid deposits, and is in equilibrium with, the a mixed 

 crystal whose composition is given by the point b, while just below 

 the temperature C the liquid forms (3 mixed crystals, much richer in 

 tin and given in composition by the point /. Thus, as the saturated 

 liquid cools through the temperature C an isothermal transformation 

 a + liquid = ft takes place. The heat evolved by this reaction is well 

 marked in the cooling curves. No uniform mixed crystals of per- 

 centages between b and I can exist. The angle D probably indicates 



