426 Prof. Bragg and Dr. Cooke on the 



result agrees with, and is an extension of, the principle 

 established in the paper already quoted. 



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

 steps of the ionization curve are more clearly shown in methane 

 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 EaC and "880 for RaA. This seems to show that the 

 stopping-power of H is rather lower than the value previously 

 given ; but the exact determination depends on the value 

 adopted for the carbon atom, which is at present the subject 

 of investigation. 



The total ionization in methane was found to be 1*165 

 times greater than in air. Initial recombination effects were 

 small, probably less than in air : experiments on this point 

 are not yet complete. 



Preparation of tJie Methane. 



The gas was prepared by acting on an aluminium-mercury 

 couple with a mixture of methyl alcohol and methyl iodide, 

 following the directions of Bone and Wheeler (J. 0. S. Trans. 

 1902, p. 511). These authors freed the methane from the 

 hydrogen present as an impurity by passing the gases over 

 " oxidized" palladium warmed to 100° C. Charitschkaff, 

 however, states (J. C. S., A ii. 1903, p. 186) that when a 

 mixture of hydrogen and methane is passed over palladinized 

 asbestos not only does the hydrogen burn, but also some 

 methane. 



In our experiment the gases issuing from the generator 

 were passed first through two vessels immersed in alcohol 

 which had been cooled to its freezing-point, roughly 160° T. 

 This cooling served to condense the vapours of iodide and 

 alcohol, and to remove also any higher hydrocarbons which 

 might have been formed. 



The mixture of methane and hydrogen passed secondly 

 through two vessels cooled in liquid air. In these vessels the 

 methane condensed to a colourless liquid, while the hydrogen 

 passed on and was neglected. 



After sufficient methane had liquefied communication 

 between the first and second pair of cooling vessels was cut 

 off', and the methane allowed to evaporate into a mercury gas- 

 holder. The methane was then recondensed as far as possible 

 by again cooling the vessels in liquid air. Part of the 

 methane was then allowed to evaporate into the gasholder, 

 and the gas coming off was pumped away. This partial 

 evaporation was repeated, and a second portion of gas re- 

 moved. In this way the hydrogen remaining in the con- 



