Chemistry and Physics. 331 



the axis is less at effective ranges for the larger shot than for the 

 smaller ones."— PHI. Mag., 34, 332, 1917. h. s. u. 



6. A New Secondary Radiation of Positive Bays. — The ques- 

 tion as to whether positive rays are capable of exciting the char- 

 acteristic X-radiation has been recently investigated by M. 

 Wolpke. The experimental method may be briefly outlined as 

 follows. A pencil of positive rays was allowed to pass through 

 a channel of circular cross-section (one cm. in diameter) and to 

 impinge upon the covering of a circular opening in a brass box. 

 Each half of this opening was covered by a double layer of 

 metal foil composed of a heavy metal (tin or lead) and a light 

 metal (aluminium). In one half of the window the heavy metal 

 of the double foil was turned toward the incident rays and in 

 the other half the light metal faced the exit end of the long 

 channel. Behind the foils the photographic plate was placed. 

 The foils used were 0016 mm., 0028 mm., and 0007 mm., in the 

 cases of tin, lead, and aluminium, respectively. Such secondary 

 cathode rays as might have been produced by the positive rays 

 and the X-rays in the channel were deflected behind the channel 

 and prevented from striking the window of the box by means of a 

 magnetic field of sufficient strength established between the 

 channel and the box. In order to eliminate the effect that might 

 have been produced by any irregularity of thickness of the two 

 foils, every test was checked by a second exposure with the faces 

 of the double foils reversed. 



The characteristic radiation of the heavy metal being more 

 intense and harder than that of aluminium, it reaches the photo- 

 graphic plate with an intensity not appreciably diminished. On 

 the other hand, the characteristic radiation of aluminium is weak 

 and soft and is absorbed to a large extent by the layer of heavy 

 metal through which it has to pass before striking the sensitized 

 surface. Therefore, if it be true that the characteristic radiation 

 of the heavy metal is stimulated by positive rays, then the 

 impression produced on the photographic plate must be stronger 

 behind that half of the window where the positive rays impinge 

 upon the heavy metal, and less strong behind the other half 

 where they first encounter the aluminium foil. The higher the 

 intensity of excitation of the characteristic rays the more pro- 

 nounced will be this difference in the strength of the impression 

 produced. These theoretical considerations were verified by all 

 of the experimental data. The author summarizes his results in 

 the following words : "1. For the first time the excitation of a 

 penetrating radiation by positive rays was observed. This effect 

 was retained on photographic plates in the case of tin and lead, 

 and it is surmised that it is the characteristic X-radiation of 

 these elements. 2. A lower limit was found to exist for the 

 voltage necessary for excitation. 3. Einstein's quantum con- 

 dition leads to the supposition that the new effect that has been 



