866 Mr. G. P. Thomson on the Application of 



(2) The method of volatizing a metal used by Dempster 



has not yet been described in sufficient detail for 

 its possibilities and limitations to be clear. 



(3) Anode rays of the type described in this paper, 

 (dt) Positive particles given off from heated salts. 



The last two methods are so similar in principle that they 

 are best considered together. At first sight the chief dif- 

 ference between them is that in the case of the anode rays 

 the local heating is produced by bombardment by gaseous 

 ions, while in the other case it is supplied by a separate 

 heating system. Gehrcke and Reichenheim fonnd that they 

 could pass from one type of rays to the other continuously. 

 Thus, if some halogen salt enclosed in platinum foil was 

 heated and used as an anode, it became the source of a beam 

 of coloured light ; if the current through the discharge-tube 

 was sufficiently increased, the rays would persist even when 

 the independent heating current was stopped. [It is, how- 

 ever, possible that the mechanism in the two cases is really 

 different though the result may sometimes be the same, and 

 it must not be assumed without trial that they would gi\e 

 equivalent results for all metals.] Of course, the necessity 

 for a separate heating current, which has to be carefully 

 insulated, is a disadvantage : but the real difference in prac- 

 tice from the point of view of analysis of isotopes is that 

 the anode rays proper require a moderately high pressure, 

 while, if the salt is heated independently, a very low pressure 

 can be used. 



This low pressure has its advantages and disadvantages. 

 The chief advantage is that it makes it possible to have a 

 very good vacuum in the camera and so get fine lines, while 

 to balance this it ceases to be possible to trace the path of 

 the rays through the gas and so see if they are properly 

 aligned on the fine tube, or indeed exist at all. It might be 

 possible to secure both by first running the anode at a fairly 

 high pressure and centering the rays, and then reducing the 

 pressure to take the photograph. There is, however, the 

 difficulty that anodes of this type do not give a good beam 

 for very long. The disadvantage from which the anode 

 rays suffer of requiring an appreciable pressure is not 

 important with elements like lithium and beryllium, which 

 are easily within the separating power of any apparatus : 

 nor would it probably be a serious drawback to the use of 

 Dr. Aston's mass spectrograph for elements of atomic 

 weights of 40 or 50 (<?. a. calcium), but it might lead to 



