Ill 



THE lONISATION CURVE OF METHANE. 



By W. H. Bragg, M.A., F.R.S., Elder Professor of Mathe- 

 matics and Physics in the University of Adelaide, and 

 W. T. Cooke, D.Sc. 



[Read June 4, 1907.] 



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

 1907) that the loss of energy experienced by the a particle 

 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 N and O 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 particles in hydrogen itself were too long for the apparatus 

 at our disposal. We therefore prepared some methane 

 (CH4), since this gas contains a large pi'oportion of hydrogen, 

 and has a convenient stopping-jDOwer. The details of the pre- 

 paration are given below. 



The accompanying figure shows the curve w4iich 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 

 words, fast a particles are less stopped by methane than slow 

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

 sult agrees with, and is an extension of, the principle estab- 

 lished in the paper already quoted. 



It is interesting to see that, as a consequence, the four 

 steps of the ionisation curve are more clearly shown in me- 

 thane than in air ; in particular, the portions due to RaA and 

 the emanation are very well separated. 



The stopping-power of methane, compared to air, is *860 

 for RaC and '880 for RaA. This seems to show that the stop- 



