On the Ionization Curve of Methane. 425 



Process (1) . But a partial separation of the gases has been 

 effected. What Lord Rayleigh proves is therefore, if you 

 complete the cycle, not inconsistent icith the truth of my state- 

 ment. TVnat he says happens in the operation which he 

 performs does in fact happen, but the work which, as he says, 

 is spent was gained in the first operation of the cycle, of 

 which he makes no mention. Xo work is done in the com- 

 plete cycle, unless in Process (1), which consists in the passage 

 of molecules along a level surface from the same pressure to 

 the same pressure. 



XXXIX. The Ionization Curve of Methane. By W. H. 

 Bragg, M.A., F.R.S., Elder Professor of Mathematics and 

 Physics in the University of Adelaide. andW. T. CoOKE,D.Sc*. 



IT has been shown by one of us (Bragg. Phil. Mag. April 

 1907) that the loss of energy experienced by the a par- 

 ticle in crossing an atom depends, in some cases at least, on 

 the speed of the particle. When the atom is a heavy one 

 there is rather more loss of energy at the higher speeds. 

 This is true of aluminium, tin, silver, and gold, in comparison 

 with each other and with air. 



It was of some importance to determine whether the prin- 

 ciple extended to gases also, and the great difference between 

 the weights of the X and atoms, on the one hand, and the 

 H atom on the other, seemed likely to furnish a good oppor- 

 tunity of settling the question. The ranges of the various 

 a particle- in hydrogen itself were too long for the apparatus 

 at our disposal. We therefore prepared some methane (CH 4 ), 

 since this gas contains a large proportion of hydrogen, and 

 has a convenient stopping-power. The detail- of the prepa- 

 ration are given below. 



The accompanying figure shows the curve which was 

 obtained as the result of the experiment. An air curve is 

 also drawn in the figure so as to make it easy to compare 

 the various ranges in the two gases. The pressure and tem- 

 perature of the air were adjusted so that the ranges of the 

 a particle from radium itself were the same in both. 



It will be seen that the ranges of the other three a particles 

 do not quite correspond. The more energetic particles go 

 further in methane than in air : thus showing that the ratio 

 of the stopping-power of methane to that of air increases 

 somewhat as the speed of the particle diminishes. In other 

 word-, fast a particles are less stopped by methane than slow 

 ones, if air is taken as the standard of comparison. This 



* Communicated by the Authors. From 'Transactions of the Royal 

 Society of South Australia.' vol, xxxi. 1907. 



