66S Mr. J. A. Orowther on the Secondary 



methyl acetate, give off secondary radiation in amounts roughly 

 proportional to their relative densities, and in character iden- 

 tical with the radiation from air, and with the primary rays. 



The second class, comprising arsine, ethyl bromide, and 

 ethylene bromide, give off relatively huge amounts of second- 

 ary radiation compared with air, and the gases o£ the first 

 type. The radiation is also of a much less penetrating- 

 character. Carbon tetrachloride stands between the two 

 classes, though approximating very closely to the first. 

 Nickel carbon yl is peculiar in giving off radiation of a very 

 soft type, in quantity not much greater than that given off by 

 gases of the first class. 



The third class is represented by stannic chloride and 

 methyl iodide. These vapours give off considerably more 

 secondary radiation in proportion to their densities than 

 gases of the first type, but much less than the gases of 

 the second class. The character of the secondary rays 

 from them is, moreover, similar to that from air under the 

 same conditions, that is to say, it is of the same hardness as 

 the primary rays producing it. Numerous experiments were 

 made with rays of as widely different character as possible; 

 but I was unable to detect any difference between the character 

 of the secondary rays from methyl iodide and those from air. 

 The radiation from stannic chloride gave similar results, but 

 always appeared to be very slightly, but still just perceptibly, 

 more penetrating than the rays from air. 



A still more important distinction between these two vapours 

 and the others in the table, is the fact that the amount of 

 secondary radiation given off by them is not a constant, but 

 varies very rapidly with the hardness of the primary rays. 

 Taking stannic chloride as an example, the amount of second- 

 ary radiation given off by it, with the bulb yvorking at an 

 equivalent spark-gap of 4 cms., was 72 times that given off 

 by air under the same conditions. This is the value given in 

 the tables. Bepeating the experiment after softening the 

 bulb to an equivalent spark-gap of 1*2 cms., the radiation 

 was found to be only 19 times that of air. This is only one 

 quarter of the amount given by the harder rays, and only 

 about twice as much as would be expected from the density 

 law. Methyl iodide gave similar results. 



None of the other gases in the table behave in this way. 

 When a balance was obtained between air and any of the 

 other gases of the first or second types, for any particular 

 hardness of the bulb, it was not destroyed either by hardening 

 or by softening the bulb. Special experiments were made on 

 carbon dioxide, from the first class, and ethyl bromide and 

 ethylene bromide, from the second, having special regard to 



