366 Proceedings of Royal Society of Edinburgh. 
and thereafter reduced to a fine state of division by planing with 
an ordinary hand plane, which was, of course, kept scrupulously 
clean. As the setting of the plane was never altered, and the 
amount of dry ice in the wet mass obtained was found to he 
subject to little variation under our experimental conditions, we 
were confident that the surface of contact between the ice and the 
solution varied little in the various experiments. This point is of 
some importance, as it is well known that the ice which separates 
from many solutions on overcooling is by no means in the same 
state of division as the ice which separates from pure water under 
the same conditions, so that the error due to the lag in attainment 
of equilibrium may vary considerably owing to this circumstance. 
The external bath by which our apparatus was surrounded was 
invariably an ice bath. It was impossible, therefore, for ice to form 
within our apparatus, whether it contained ice or solution. By 
this means a possible source of error was avoided, the formation, 
namely, of a skin of ice round the bulb of the thermometer, which, 
according to the experiments of Wildermann, greatly affects the 
apparent temperature of equilibrium (ibid., xv. 358). 
Apparatus. 
The general arrangement of our apparatus is shown on p. 367. 
The vessel used to contain the experimental liquid was a Dewar 
vacuum tube, which was closed by a rubber stopper perforated by 
three holes. Into these holes were fitted three tubes, which served 
as collars. The central tube C was of copper and was wide enough 
to admit the thermometer T. The other two tubes were of glass, 
and through them passed the filter F and stirrer S respectively. 
The stirrer was made of light glass tubing bent horizontally to 
a circular form at its lower extremity. On the inner surface of 
the circular portion was a number of small holes, through which, 
by means of a hand blower attached to the upper portion of the 
stem, fine jets of air could be blown, thereby supplementing the 
mixing brought about by the vertical motion of the stirrer as a 
whole. The filter tube was made of narrow glass tubing widened 
to a conical shape at its lower end. The filter, which consisted of 
a small disc of filter paper supported between two discs of muslin, 
