BY THE ACTION OF A RADIO-ACTIVE SUBSTANCE, ETC. 
269 
short pieces, S x and S 3 were saddled on to it, each of them surrounding one of the 
windows. A quartz-plate, Q, was fixed to the end of S x by means of sealing wax, 
which made the joint air-tight, and a piece of wire gauze, having the same curvature 
as A, was placed in the window W l5 completely filling it. A piece of zinc, Z, of the 
same shape and size as the piece of brass which was cut out of the window W : , was fixed 
to a brass rod, It, which passed through the ebonite disc D. The disc fitted tightly 
into the end of S 2 , and the joint was made air-tight. The zinc did not touch the 
tube A, so that its potential relative to A could be varied as desired. When ultra¬ 
violet light passes through the quartz and the gauze, it falls on the zinc, and negative 
ions are produced at the surface of the metal. Some of these ions can be sent into a 
stream of air passing along A by lowering the potential of the zinc relative to A. 
For this purpose a small battery, IT, was insulated and its positive terminal was 
connected to A and its negative terminal to It. 
A spark between two aluminium wires was used as the source of ultra-violet light. 
The apparatus for producing the spark was contained inside a box covered with lead, 
with a small opening at through which the light from the spark fell on the quartz- 
plate Q. One of the terminals of the secondary of a Ruhmkorff coil was connected 
to the outer coating of a Leyden jar, and the other terminal to the inner coating. 
The jar was charged by the coil, and the discharge took place across the spark-gap 
formed by the two aluminium wires. The air in the neighbourhood of the spark 
becomes positively electrified, so that it was found necessary to pack wool round the 
tube S : to prevent the electrified air from coming into the neighbourhood of the rod F 
and the wire connecting it to the electrometer. When this precaution was taken it 
