86 Mr. McClung on the Effect of Temperature on the 
insulated from the other part of the plate. The outer part of 
the plate acted as a guard-ring and was connected to earth, 
while the central disk could be connected to the electrometer 
by the key K when desired. The rays passed between these 
plates on the way to the brass cylinder, and ionized the air 
between them. If the intensity of the rays remained con- 
stant the saturation-current between these plates should be 
constant, and any variation in the intensity of the rays should 
be shown by a corresponding variation in the current. After 
the readings had been taken on the air in the cylinder BC, the 
connexion at K was transferred to the electrode F, and 
the rate of leak in the standard apparatus measured. By this 
means it could be determined whether any variation, which 
might occur in the rate of leak between the electrodes in the 
cylinder BC, was due to a variation in the intensity of the 
rays or to some other cause. This therefore served as a 
test of the rays. | 
In observing the amount of ionization, as indicated by the 
deflexion of the electrometer-needle, it was found that as the 
temperature of the air increased the deflexions decreased, 
and that they varied in the inverse ratio to the absolute 
temperature. Now we must take into account here the fact 
that as the temperature of the air is increased its density de- 
creases in the inverse ratio, since the gas is quite free to expand 
into the outside air. It was shown by Perrin* that the amount 
of ionization produced in a gas is proportional to the pressure 
of the gas, and this result was later confirmed ty Rutherford 
and McClung+. ‘Therefore, in the present instance, when 
the density of the gas decreases, there would be a corre- 
sponding decrease in the amount of ionization produced, due 
entirely to the change in density. To determine the effect 
on the ionization due to a change of temperature alone, a 
correction must be made for the change of density of the gas. 
In making these experiments then it was found that the 
decrease in the amount of ionization which took place as the, 
temperature rose was just the amount of decrease which 
would occur on account of the decrease in the density of the 
gas, and when the necessary correction was made for the 
change of density no alteration in the amount of ionization 
was produced by the variation of the temperature itself. In 
other words, if the density of the gas were kept constant the 
amount of ionization produced by rays of given intensity 
would be independent of the temperature of the gas. A 
series of readings is given in Table I. as a sample of the 
results which were obtained. 
* Annales de Chimie et de Physique, xi. p. 496 (1897). 
+ Phil. Trans. exevi. (1901). 
