512 
HON. U. J. STRUTT ON THE CONDUCTIVITY 
The procechire in investigating each kind of radiation was as follows :—First, the 
rate of leak was taken in air at different j^ressures; the limit of pressure within 
v/hich the rate of leak was sensibly proportional to the pressure was thus ascertained. 
And in the subserpient measurements care was taken to he well inside this limit. 
Fig. 2. 
In comparing the different gases, air was in all cases taken as the standard. The 
rate of leak in air was measured at a pressure such as to give this rate a convenient 
value. The air was then removed hy the water-pump, and the gas under investiga¬ 
tion admitted. The apparatus was several times exhausted and refilled to ensure 
jiurity. Finally, the pressure was adjusted to give about the same rate of leak as 
that previously measured in air, and the exact rate carefully determined. If p p' 
were the pressures of the gas under investigation and of the air respectively, ^ i' the 
observed rates of leak, then the relative saturation conductivity of the gas was given 
, air at the same pressure and under radiation of the same stn 
being taken as unity. 
The electromotive force used was 200 volts, amply sufficient to produce saturation 
under the conditions of the experiments. 
In investigating the vajjours of volatile licpiids, such as methyl iodide, it was 
necessary to take care that the vapour shoidd not be so nearly saturated as to deviate 
sensibly from Boyle’s law. The rates of leak through vapours were accordingly 
taken at a ])ressure of not more than half that wliich would have been in equilibrium 
over the liquid at the same temperature. These vapours gave large conductivities, 
and the smallness of the pressure at which it was necessary to Avork did not cause any 
by the fraction 
I p 
moonvenience. 
