of Secondary Cathode Rays produced by X-Rays. 41 



produced in the gold by a certain beam of X-rays from R x 

 to the incidence secondary radiations produced by the same 

 beam after traversing the ionization-chamber. In the light 

 of the results obtained in a previous paper*, and owing 

 to the fact that the ionization-chambers used in the present 

 work were only two fifths as long as those used before, 

 we may assume that the secondary X-rays from the metal 

 produced a negligible amount of ionization in the chamber 

 compared with that produced by the cathode rays from the 

 metal. 



No data are readily obtainable in regard to m, but it is 

 known that the amount of cathode rays given out by 

 aluminium is very small compared with the amount given 

 out by metals of high atomic weight such as gold and 

 silver. Unless the ratio of emergence to incidence radiation 

 is far different in aluminium from what it is in gold and 

 silver, which is unlikely, the difference between m and n 

 will be a quantity which we may neglect, and we may write 

 for a sufficient approximation 



e/i = a/b. 



The determination of e/i, as above described, was always 

 repeated with the two plate-holders reversed with respect 

 to the ends of A, which should give the same result if both 

 plates were the same. The values obtained, however, in the 

 two cases differed very widely ; much more than could be 

 accounted for by the experimental errors. The only ex- 

 planation seemed to be that, owing to dirt on the surface or 

 variations in the thickness of the metal foils, one plate was 

 more efficient in giving out cathode rays than the other. 

 If this were the case both the incidence and emergence 

 rays should be affected in the same ratio, and the geometric 

 mean of the two determinations of e/i should give the same 

 result. To test this different pairs of plates were made 

 and also more foils were added to the same plates, but no 

 matter what the discordance was between the two values 

 of e\i for a given pair of plates, the geometric mean of the 

 value found when one plate was on the front of A and the 

 other on the back, and the value when these positions were 

 reversed, always gave the same result within the limits of 

 error of the experiment. 



This ratio e/i does not, how 7 ever, give the true ratio of 

 emergence to incidence effect for the same intensity of 

 exciting rays, owing to the fact that these were absorbed 



* Am. Jour. Sci. vol. xxiv. p. 285 (1907). 



