On the Constitution of Copper- Tin Alloys. 323 



wholly arrested, or forced to occur on so minute a scale as to be 

 invisible. This is well seen in the photographs 4, 5, 6, of the paper 

 published in February, 1901.* 



Inasmuch as the evidence for our statements consists largely of a 

 photographic record of the structure of the various chilled alloys it 

 cannot be given in detail in the present paper, but some of this 

 evidence has already been published in our two papers referred to 

 above. We shall content ourselves at this moment with offering such 

 an explanation of the annexed diagram as will make our conclusions 

 intelligible. 



In the diagram the atomic percentage of tin in the alloy, and also the 

 percentage by weight of tin, are stated at the top. The atomic per- 

 centage, being the horizontal ordinate in the diagram, appears as a 

 scale of equal distances, and therefore the percentage by weight is 

 indicated on a gradually increasing scale ; but by intrapolation, any 

 intermediate percentage can easily be found. The margin of the 

 figure on the left corresponds to pure copper, that on the right to 

 pure tin. The temperature scale is plotted vertically in degrees 

 Centigrade. 



In such a diagram, if we travel down a vertical line from the top to 

 the bottom, we are considering the same alloy (so far as total per- 

 centage is concerned) at different temperatures, and, therefore, in 

 different states of aggregation. Each closed compartment of the 

 diagram corresponds to a different state of aggregation, and the 

 phases making up the aggregates are indicated for each compart- 

 ment. We can, therefore, from the position of the point, determine 

 at once the state of the alloy. 



The following phases occur : 



(1) Liquid. 



(2) (3) (4) Mixed crystals of three types, a, /?, y. 



Each of these is a uniform solid solution containing copper and tin, 

 but of variable percentage as in the case of a liquid solution. 



(5) The body E', which composes the whole alloy at the point E'. 

 This phase is certainly in many cases the pure compound Cu 3 Sn, but 

 there may be compartments in which the phase which we shall still 

 call E' is a solid solution of Cu 3 Sn and some other body. 



(6) The body H, which first appears in certain alloys when they 

 have cooled to a temperature of 400 C. There are various reasons, 

 some of which we shall give at a later period, for believing that pure 

 H is the compound CuSn, but we have, not yet obtained conclusive 

 proof that it habitually crystallises in the pure state. 



(7) The solid crystallising on the branch IK, which must be pure tin. 

 Each of the above six solids can exist in contact with the liquid 



at appropriate temperatures and concentrations. 

 ' Boy. Soc. Proc.,' vol. 68. 



