NUCLEAR CONDENSATION OF CERTAIN ORGANIC VAPOURS. 
449 
indicated in the figure. A thin film of the liquid also spreads over the interior 
surface of T, and so it is to be expected that the air in T is quickly saturated with its 
vapour. The beam of light, obtained from an incandescent mantle, I (see also the 
upper part of fig. 4), with a slit about 1 cm. wide, was focussed inside the test-tube T. 
All other light was excluded and a black background obtained by means of black 
velvet. Most of the drops formed by an expansion could be readily seen on looking 
from E. 
Another expansion chamber was used to determine the sign of the charge of the 
more efficient ionic nucleus. It is drawn to scale in fig. 3. K is a portion of a glass 
tube, 3 - l cms. in length and 4'2 cms. in diameter, with ground edges. The ends of the 
chamber are thick aluminium discs, A and B. The joints between these and the glass 
cylinder K, and between L and B, are made by means of the rubber rings R. Three 
bolts such as N, insulated from A by vulcanite collars, press the aluminium ends A 
and B and the connector T against the rubber rings R. The joint obtained in this 
way is air-tight and remains so. The chamber is divided into an upper and lower 
half by a thin aluminium diaphragm, D, of 3'6 cms. diameter, supported by three 
projections pressing against E. Thus there is an annular space 3 mm. wide 
between the edge of the diaphragm and the wall E. In order to make the lower half 
of the chamber as similar as possible to the upper, a second diaphragm, C, of the same 
size as D, is placed over the outlet in the end B, and is connected to B by means of a 
spiral S of platinum wire. A ring of thin aluminium, E, 1 cm. in width, was placed 
symmetrically around D, in order to reduce the intensity of the secondary rays from 
the walls of the vessel, for it is here that the Rontgen rays (mentioned later) are 
incident. The ends A and B, &c., were made of aluminium with this object, and 
because the liquids used, when dry and pure, were found not to have any chemical 
action on aluminium. It it necessary to have the central diaphragm D electrically 
connected to the exterior of the vessel. A tapered aluminium wire filled a hole in the 
glass wall at H and was cleated down on the inside and sealing waxed on the outside. 
The cleated end is in contact with E, which supported D. This gives a joint which 
protects the sealing wax from the action of the liquids used. 
Rontgen rays (see fig. 4) from a large bulb and coil pass through a horizontal slit 
between two lead blocks 2'8 cms. thick and 0 - 5 cm. apart. The narrow band of rays 
thus obtained passes through the ionisation chamber above and below the diaphragm D. 
The relative positions of the bulb, slit, and expansion chamber were finally adjusted 
by means of a barium-platino-cyanide screen placed at Y. 
With these’adjustments made, the shadow at Y of the diaphragm edge-on formed 
a dark line across the middle of the “ image ” of the slit. 
Separation of Positive and Negative Ions. 
The terminals of a two-volt cell were connected to a potentiometer wire (U of 
fig. 4) of 2000 ohms resistance (with contacts at every 20 ohms), which was connected 
VOL. CCVIII.—A. 3 M 
