Condensation of the Radioactive Emanations. 573 



calculation of the number actually present, but an estimate 

 of the order of the number present can be deduced from the 

 following considerations. The radiation from the thorium 

 and radium emanations consists, as far as it is known, entirely 

 of a. rays. The view has recently been put forward that these, 

 in the case of radium, consist of projected particles of the 

 same order of mass as the hydrogen atom, carrying a positive 

 charge, and travelling with a velocity about 1/10 the velocity 

 of light. It is extremely likely that the radiations from the 

 two emanations are quite comparable in character, and produce 

 in their passage through the gas a similar number of ions. 

 From results deduced from experiments made in an attempt 

 to measure the charge carried by the a. rays there is no 

 doubt that each of these projected particles produces at least 

 10 4 , and possibly 10 G , ions in its path before being absorbed 

 in the gas. For the present purpose, 10' ions will be taken 

 as a probable value. The electrometer employed readily 

 measured currents of 10~ 3 E.S. units per second. Taking the 

 charge on an ion as 6 x 10~ 10 E.S. units, this corresponds to 

 a production of l"7xl0 6 ions per second, which would be 

 produced by 17 expelled " rays " per second. Each radiating 

 particle cannot expel less than one ray, and may expel more, 

 but it is likely that the number of rays expelled by a particle 

 of the thorium emanation is not greatly different from the 

 number expelled by a particle of the radium emanation. The 

 view will be developed more generally in a subsequent paper *, 

 that the decay of activity of a radioactive substance is caused 

 by the number of particles present diminishing owing to their 

 changing into new systems, the change being accompanied by 

 the expulsion of rays. From the law of the rate of decay 

 \ t = Y o e~ kt , on this view \N particles change per second 

 when N are present. Therefore to produce 17 rays per 

 second \N cannot be greater than 17. Since in the case of 

 the thorium emanation X equals 1/87, it follows that N cannot 

 be greater than 1500. The electrometer used therefore de- 

 tects the presence of about 1500 particles of the thorium 

 emanation, and since, in the static method, the volume of the 

 condensing spiral was about 15 c, c, this corresponds to a 

 concentration of about 100 particles per c. c. An ordinary 

 gas at atmospheric pressure probably contains 10 20 particles. 

 On this estimate therefore the thorium emanation could have 

 been detected if it possessed a partial pressure in the con- 

 densing spiral of 10 -18 atmosphere. It is thus not surprising 

 that the condensation-point of the thorium emanation is not 

 * Infra, p. 576. 



Phil. Mag. S. 6. Vol. 5. No. 29. May 1903. 2 Q 



