14 Prof. Challis's Theory of the 



The numbers in the last column show very clearly : — 



(1) That the loss of silver is never even distantly equivalent 

 to the amount of hydrogen developed; 



(2) That the knobs suffer greater losses, under otherwise like 

 conditions, the oftener they have already served for experiments 

 — that is, the more their form has been already broken up and 

 become adapted to the passing arc of light ; 



(3) That in experiments of longer duration (where of course 

 a brief interruption was always made when the knobs began to 

 approach the temperature of fusion) the losses are more consi- 

 derable ; that is, higher temperature of the knobs determines 

 greater losses ; 



(4) That the positive electrode of silver suffers smaller losses 

 when the opposite parts of the negative plate are dissimilar in 

 constitution ; for the experiments marked with an asterisk were 

 made with a copper plate which had been first prepared at the 

 turning-lathe, and presented concentric higher and deeper por- 

 tions. In the commencing experiments the silver knobs were 

 frequently carried transversely over these unevennesses. 



(5) That the knobs taken for the positive electrode give 

 greater losses. 



It does not, then, depend exclusively on the chemical nature 

 of the electrodes and the intensity of the current, but much more 

 on manifold circumstances, how great the quantity of the disin- 

 tegrated material shall be. 



For these experiments, if they are to be performed in the pre- 

 ceding simple form, only non-oxidable electrodes can with ad- 

 vantage be used. I have not on this account deemed it necessary 

 to follow up the experiments further with expensive metals of 

 this sort, because those communicated are quite sufficient to de- 

 finitively cut off all supposition of any simple connexion be- 

 tween the intensity of the current and the disintegration of 

 the electrodes. 



Aachen, April 4, 1873. 



III. A Theory of the Source of Terrestrial Magnetism. 

 By Professor Challis, M.A., F.R.S., F.R.A.S.* 



IT may facilitate the understanding of the theory which is 

 the subject of this communication if I begin with indi- 

 cating the writings in which I have already discussed the 

 theory of magnetism generally, and make some remarks on the 

 views included therein as to the specific character of the 

 earth's magnetism. 



* 



Communicated by the Author. 



