508 
HON. R. J. STRUTT ON THE CONDUCTIVITY 
ions by the rays. VVlien the current is saturated, the ions travel across from one 
electrode to tlie other so quickly that there is no time for recombination to take place 
to any appreciable extent. Thus the current measures the amount of ionisation 
produced, supposing the strength of radiation to remain constant. In order to 
determine whether or not the radiation was sensibly absorbed, the conductivity was 
olrserved at different })ressures. When the conductivity was proportional to the 
pi’essure, it was concluded that no sensible absorption took place in the layer of gas at 
any pressure within the range of tire experiment. When, on the other hand, the 
conductivity did not increase so fast as the law of proportionality to the pressure 
wordd require, it was clear that the absorption was appreciable. In such a case, the 
gas was rarefied far enough to ensure that the law of proportionality should he 
obeyed. 
§ 2. Ro/lio-active Substances. 
I'he radiation from various ludio-active Irodies has been investigated. These 
included— 
(1.) A preparation obtained from de Haen, of Hamburg (see ‘ Wied. Ann.,’vol. 68, 
p. 902). This substance, as a few simple chemical tests showed, consists principally 
of liarium carbonate, and, no doubt, its very strong radiating power is due to the 
presence of the new metal, radium, discovered by Madame Curie in pitchblende, 
wliich is separated, together with barium, in the analysis of that mineral. 
This radium preparation gave out radiations of at least two distinct kinds : one 
easily absorbable l>y solids or gases, and, as Curie has shown (‘ Comptes Eendus,’ 
vol. 130, p. 73), not deflected by magnetic force. The other, more penetrating, and 
deflectable by the magnet. The relative conductivities in gases due to each of these 
kinds has been investimited. 
Since, when investigating the conductivity due to the non-deflectable kind, the 
other variety were also present, it is important to inquire whether the proportion is 
large enough to vitiate tlie results. 
In Curie’s paper the following numbers are given, showing what proportion of the 
conductivity is due to the non-deflectable I'ays at various distances from the 
source ;— 
I 1 )istaiice ceiitim. . . 7 • 1 0-9 G • '> G • 0 5 • 1 .3 ■ 4 
I Percentage .... 0 0 11 33 5G 74 
The alisorption of the air, of course, accounts for the small proportion at long 
distances. A rough extra-polation from these numbers shows that close up to the 
