OSMOTIC PRESSURES DERIVED FROM VAPOUR-PRESSURE MEASUREMENTS. 321 
reduced to small dimensions if care be taken that the full air stream is not turned on 
before the apparatus has come to the bath temperature ; on the other hand, we found 
that taking the apparatus out of the bath without any preliminary treatment* 
caused a large error owing to the fact that the connecting tubes (and other air 
spaces) cooled more rapidly than the liquids, and so acted as condensers on which 
moisture was deposited ; this trouble was eventually overcome by running cold water 
into the bottom of the bath (so as to displace the hot water) until it just reached the 
level of the vessels. In this manner the liquids were caused to cool earlier than the 
connecting tubes. 
Another Possible Source of Error is that Due to Diffusion. —A small amount of 
diffusion of water vapour from the solvent to the sulphuric acid takes place during 
the time the apparatus is being set up and taken down ; the total time taken for 
these operations is roughly about four hours, and calculation shows that at 12° C. 
not more than 0‘0002 gr. of water would come over. 
The Vessels. —Four different sets of vessels have been used in this research, and 
for a proper understanding of the objects for which they were made, a brief 
description follows. 
In the earlier part we used the set of vessels (with their platinum joining 
tubes, &c.) and the method of oscillating the platform on which they stand in the 
bath described in ‘Phil. Trans.,’ A, vol. 209, pp. 177-203. In the same communi¬ 
cation it will be noticed that one experiment (p. 188), where there had been no 
oscillation, gives a result that scarcely differs from the others. This led to a number 
of test experiments (details need not be given) which satisfied us that oscillation, 
anyhow under certain circumstances, was not essential. 
We will call this set, when oscillated, Apparatus Aa; and without oscillation, 
Apparatus Ab. It is to be noted that from now onwards oscillation was given up. 
In the next set (Apparatus B) which followed the general lines of Apparatus A, 
but was unfortunately constructed of soft Jena-glass, three changes were made : 
(l) the capacity of the vessels was largely increased (the diameter of the tubing 
forming the horizontal branches is now 1'7 cm.) to ensure that, for a dilute solution 
such as weight-normal sugar, the loss of the water vessels would be large enough 
to reduce the probable weighing error to 1 part in a 1,000 ; (2) the dipping tube, 
that is the tube joining the water and sulphuric acid vessels, was now increased in 
diameter from 1 mm. to 6 mm ; (3) the platinum joining tubes were replaced by glass 
ones as shown in fig 4, which is a vertical cross-section through the centre of the 
vessels ; A, B, and C are ground and lubricated joints, drowned in mercury; whilst D 
is a silver sleeve which was added when constructing the quartz-glass apparatus to 
be described. The object of this new form of joint is to enable the observer to weigh 
the inverted U-tubes E so as to find out whether water has been deposited in them. 
We would state here that normally water is only found in the U-tube joining the 
* The precautions to be taken when working at 0° C. will be obvious. 
2 U 2 
