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.
