( 288 ) 
of cutr substances was in general sufficient for the purpose in view. 
Yet it is usefiul just to consider what might be the influence of 
impurities on the extent and the sense of the change of the plait- 
point temperature with the pressure. We expect a priori that this 
influence will be small, unless we should just happen to be in the 
neighbourhood of a maximum or minimum of plaitpoint temperature; 
in this case the influence of the pressure itself is very small, and 
even traces of impurities might modify the sense of the observed 
The following examples, chosen as unfavourable as possible, show: 
the first the slight influence which the most common contamination 
(water) has on the value of dtfdp, the second how in a special case 
a trace of impurity has already sufficed to change the sign of this 
differential quotient. 
I, Methyl alcohol and cyclohexane Kr. T. — 
59.0 + 0.03 
do contaminated with water 92.8 + 0.06 
II. Methylalcohol and methylene iodide, 
contaminated with traces of iodine 
after a reaction between the free iodine and 
the mercury of the Cailletet-tube, in which 
green mercuroiodide is formed 
93.8 — 0.004 
82.9 + 0008 
As in our present investigation we only purposed to deteimine 
the direction and the order of magnitude of the change of 
point temperature with the pressure, the method used was simp i 
as much as possible with a view to the quickness of the measuremen 
Mixed in the proper proportion the substances to be inveshga 
were put in a Cailletet tube, which was provided with a long 
capillary; after the air had been expelled as thoroughly as P 08 ^ ’ 
the capillary was sealed. The tube was surrounded by a jac 
filled with water or paraffin, which was brought at the 
temperature by an electrical heating apparatus; the bath was * 
by the hand; the temperature was read on a GeisslkR thermn ^ 
graduated in degrees, and the pressure was determined by a man ^ 
of Schaffer & Budenbkrg divided into atmospheres. |0 
tube was provided with an electro-magnetic stirrer, and in 
reach the state of equilibrium as easily as possible, the reaa 
