(5 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
the li(j[nuliis represent liquid states, so all points immediately below the solidus 
represent wholly solid states of the system."^ Points between the solidus and the 
liquidus stand for states of the system in which it is a mixture of solid and liquid. 
The determination of the solidus hy experiment is much more difficult than the 
determination of tlie liquidus. By stirring an alloy during solidification, and noting 
the temperature at which the stirrer becomes immovable, one gets a very rough 
ap})roxlmation to a point on the solidus. A measurement from the cooling curve 
looks more promising, but is liable to prove very misleading. For example, in the 
cooling curve of Sn 12, solidification begins at the upper flat near 870°, but is far 
from complete at the end of tliat flat, continuing, of course with evolution of heat, to 
a little way below the end of the second flat at 790°. This temperature is a point on 
the solidus. Such cooling curves as those of Sn 4, Sn 17, or Sn 26 would leave the 
position of the solidus still more uncertain. It would seem that, assuming perfect 
equilibrium at all stages, the solidus must be identical with the “melting-point” 
curve, a ])oint on the latter being defined as the temperature at which an alloy which 
is being very slowly heated begins to melt or form liquid. Here, again, on account 
of the impossibility of maintaining true equililjrium between the various crystals in a 
heterogeneous alloy, the experimental error would be large. We have attempted to 
determine points on the solidus l^y very slowly warming small rods of allo}^ and at 
the same time subjecting them to a slight strain, the temperatui'e at which the rod 
breaks being taken as that of the solidus. These experiments have not given very 
consistent I'esults, l)ut they have been valuable in showing the high temperature up 
to which many of tlie alloys I'etain their rigidity. 
We have found that by firr the most satisfactory method of determining points on 
the solidus was througli the study of polished and etched sections of chilled ingots of 
alloy. These show quite plainly how much of the 
ingot v’-as liquid at the moment of chilling, and by 
chilling at successively lower and lowei' tempera- a 
tures one can determine the temperature at which 
the whole ingot has become solid. In our Plate 11 
the branches of the solidus were determined by 
such a study of the chilled alloys ; for this purpose | 
many chills were made in addition to those descriljed I 
m bection III. 
Professor Boozeboom defines the solidus diflerently, 
although the same curve is ultimately arrived at. 
As a clear understanding of Professor B ooze doom’s 
view is essential for the argument of the present paj)er, perhaps we mav be excused 
for restating it here. 
* It is more accurate to say that if we travel from A to I along the 6ohdu$, points on onr right 
represent wholly solid states. 
Concentration 
