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LXIX. On the Cadmium- Vapour Arc Lamp. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN the March number o£ the Philosophical Magazine 

 Mr. F. Bates contributes a paper on a new cadmium- 

 vapour arc lamp, in which a similar lamp described by me 

 about four years ago (Proc. Phys. Soc. xxviii. p. 94, 1916) 

 is subjected to adverse criticism. It is stated that " the im- 

 purities introduced into the lamp " by my method of filling 

 " effectively prevent obtaining a relatively pure intense 

 cadmium spectrum." It is not clear what is meant by this 

 statement. The only impurity which I suggest introducing is 

 zirconia, and zirconia sufficiently pure not to affect the spec- 

 trum of a cadmium-vapour arc is neither difficult to obtain nor 

 to prepare. If the impurities are " introduced " in the process 

 of filtering the metal in a vacuum, the fault obviously lies 

 with the operator and not with the method.- If, however, 

 what is meant, is that metallic impurities such as zinc are 

 not removed, this is obviously true, but holds also for the 

 method of distillation recommended by Bates. The only 

 satisfactory method of avoiding such impurities is to start 

 from pure cadmium. 



It is further stated that the zirconia does not sufficiently 

 prevent adhesion of the metal to the glass, and that " if the 

 lamp does not crack upon the first solidification ". . . . " upon 

 cooling a second time the lamp was invariably cracked." In 

 reply to this, I must point out that I do not remember a single 

 case in which a lamp which I have filled myself has failed 

 from this cause. I have also obtained evidence from two 

 independent users of my lamps. One user states " I have 

 had one of your lamps in continuous use since 9/6/16 ... It 

 is still working,— it has been in very frequent use, upon 

 some occasions for 2 or 3 hours on end ; the glass has now 

 become somewhat opaque to the shorter wave-length radiation. 

 Other lamps that we have used have not had the same length 

 of life as the one I have used myself, but the breakdowns have 

 not been due to the cause stated by Mr. Bates." 



The second user states that his lamp has been in use for 

 about three years. " It has been heated up at least 500 

 times, and has burnt for over 600 hours. ' r 



It is obviously not possible for me to state positively what 

 were the reasons for Mr. Bates's failures. Insufficient 

 thickness of the quartz-glass or too coarse zirconia might be 



