Chemistry and Physics. 499 



6. Secondary Cathode Mays Emitted by Substances Exposed 

 to y-JRays. — R. D. Kleeman concludes that the y-rays from 

 radium consist principally of two groups of rays, the constituent 

 rays of each group differing not much from one another in their 

 properties. The rays of one of the groups are more efficient in 

 producing secondary cathode radiation from aluminium, sulphur, 

 iron, nickel, zinc, and tin, than from lead, and are all more or 

 less easily absorbed by each of these substances excepting lead, 

 the absorption by lead being much less. The rays of the other 

 group are more efficient in producing secondary cathode radiation 

 from lead than from the other substances, and are more easily 

 absorbed by lead, mercury, and bismuth, than by any of the 

 other substances. There is also a third, apparently weak group 

 of rays which is most efficient in producing secondary radiation 

 from carbon. This group of rays is less easily absorbed by the 

 above mentioned substances than either of the other groups. — 

 Phil Mag., Nov. 1907, pp. 618-644. j. t. 



7. Secondary Rbntgen Radiators from, Gases and Vapors. — 

 Careful investigation of this subject was made by Barkla 

 (Phil. Mag. v, 1903, vii, 1904), who concluded that: 



(1) All gases, when subjected to X-rays, are a source of sec- 

 ondary radiation. 



(2) The absorbability of the secondary radiation is, within the 

 limits of experimental error, the same as that of the primary pro- 

 ducing it. 



(3) For a given primary radiation, the intensit}^ of the second- 

 ary radiation is proportional to the density of the gas from which 

 it proceeds. 



(4) The ratio of the intensities of the primary and secondary 

 beam is independent of the hardness and intensity of the primary 

 rays. 



Barkla's experiments were performed on few gases and not of 

 a varied type. Mr. J. A. Ceowthek has repeated the experi- 

 ments with a large number of gases — of varied atomic weights — 

 and confirms Barkla's results. — Phil. Mag., Nov. 1907, pp. 

 653-675. J. T. 



8. Abrupt Limit of Distance in the Poioer of the Positive Pays 

 to Produce Phosphorescence. — It has been found by the various 

 observers that the a-rays from polonium and radium lose their 

 power abruptly of producing phosphorescence and of affecting 

 the photographic plate. Dr. Jacob Kunz, reflecting that the 

 positive ra,ys are similar in nature to the a-rays of the radio- 

 active elements, was led to believe that the positive rays would 

 also show an abrupt falling off in a manner similar to the a-rays. 

 The tube used for the production of the canal or positive rays 

 was enclosed in a larger tube, exhausted to the same degree : 

 and the inner tube by means of spiral springs connected to the 

 electrodes could be moved to varying distances from a willemite- 



Am. Jour. Sci. — Fourth Series, Vol. XXIV, No. 144. — December, 1907. 

 35 



