64 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
The first point that strikes one in the copper-tin curve is the rapid way in which 
the steepness increases with increasing percentage of tin. If we bear in mind the 
fact that, on account of the thermometer lag, so important where the curve is steep, 
the true equilibrium curve from 5 to 15 atoms of tin may lie perceptibly above our 
freezing-point curve, while, from the character of the freezing point at 15o atoms, 
the two curves must agree at this point, we can conclude that at 15 the true curve 
may be almost a vertical line. A not improbable explanation of this feature of the 
curve is to suppose that the tin is largely combining with the copper to form mole¬ 
cules, such as SnCug or SnCu^. As can he proved by the use of this assumption in 
equation (2), the view that the body in solution is SnCug agrees very well with the 
shape of the curve up to 15 atoms of tin. But we do not attach much importance to 
this numerical agreement, as the assumption of the existence of molecules of SnCu^, 
accompanied by some dissociation, would give an equally good agreement. 
At 15'5 atoms of tin our curve changes its direction, and becomes a straight line; 
the freezing point of each alloy is now marked by a steady temperature, lasting for 
some time. In the figure this point appears to be a triple point, as from 13 "5 atoms 
to 15'5 we observed two freezing points, one a slightly-marked one on the steep part 
of the curve, the other a very steady temperature identical with the single freezing 
point at 15'5. We think that the horizontal line of freezing points thus obtained 
represents the moment for each alloy when, through separation of solid matter at and 
below the upper freezing point, the still liquid portion attains the composition of 
15*5 atoms. 
The soft precipitate which forms at, or soon after, the freezing point on the upper 
branch as far as 15*2 is very unlike the abundant finely-gritty precipitate of these 
lower freezing points, and of the alloy as far as 18 atoms or beyond. 
From 15*5 to 20 atoms the freezing point is plainly marked by a constant 
temperature lasting for some time; and all the points lie on a line in which we 
cannot see any curvature. This line ends exactly at 20 atoms, and it will be seen 
that we have studied this region very minutely. 
Below the lower half of this line we find a horizontal line of second freezing points, 
causing the appearance of a triple point at, or near, 20 atoms. Each of these lower 
freezing points occurs after a good deal of solid matter has already formed. The 
alloy sets to a solid mass at the lower point, and this freezing point is marked by a 
period of absolutely constant temperature. We have no doubt that these lower 
points indicate the moment when the still liquid alloy has reached tiie composition 
of 20 atoms of tin. 
From 20 to 25 atoms the curve is very nearly, but not quite, straight, and the fall 
in temperature caused by the increase of tin from 20 to 25 atoms is comparatively 
slight, being less than 20°. The freezing point of each alloy is marked by a prolonged 
period of constant temperature. The freezing points are like those of the eutectic 
state, and each alloy may be said to solidify at a constant temperature, as if the 
