Intelligence and Miscellaneous Articles. 319 



magnetic action has been propounded. This " new theory " 

 is at least as old as Becquerel (1850), and has been investi- 

 gated by almost everybody who has worked at diamagnetism, 

 and they are practically unanimous in the conclusion that 

 diamagnetic actions cannot be explained by the paramagnetism 

 of the air. 



These undisputed experiments prove that such a theory as 

 BecquereFs would require a vacuum to be paramagnetic. 

 This is a new nomenclature, and does not make the old one 

 absurd ; it involves practically what Mr. Parker says is non- 

 existent, namely one of the theories of diamagnetism. But 

 all this is too well known to be worth wasting words on. 



As to the second curious statement of Mr. Parker's, 

 namely that no theory of diamagnetism exists, that is equally 

 extraordinary. Chapter xxii. of Maxwell's pretty generally 

 known Treatise is headed " Ferromagnetism and Diamag- 

 netism explained by Molecular Currents," and gives an in- 

 teresting description of Weber's theory of Diamagnetism in 

 § 838. If Mr. Parker had exercised his ingenuity in explain- 

 ing how Weber's theory, which, being a possible theory can be 

 reconciled with Thomson's difficulty, it would have been more 

 instructive than a dissertation on the absurdity of what undis- 

 puted experiments show to be real. If the opportunity had 

 arisen, it would have been only kind to Mr. Parker to have 

 referred the paper back to him for reconsideration before 

 publishing it. Geo. F. Fitzgerald. 



ON HOMOLOGOUS SPECTRA. 

 To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Cardiff, August 20, 1891. 



Will you kindly allow me to say a few words in reply to the 

 recent article by Professor Hartley in your Magazine (April 1891), 

 which referred almost entirely to a previous paper of mine. 



I regret that, in reading Professor Hartley's papers on Homo- 

 logous Spectra, I failed to see his statement concerning the 

 importance of noticing the physical properties of the lines of the 

 various spectra. In comparing, however, the lines of magnesium and 

 zinc, which he considers to be homologous, I saw at the most only the 

 faintest resemblance. This may be due to the fact that the photo- 

 graphs which I used were of the arc-spectra, while his were of the 

 spark-spectra. I had sets of both at my disposal ; but the arc- 

 spectra ones were so superior in every way that I selected them. 

 As I said in my previous paper, I think the arc-spectrum photo- 

 graphs are preferable in looking for homologous lines ; not because 

 the spark-spectrum is variable, as Professor Hartley thinks I meant, 

 but because of uniformity of conditions, sharpness of lines, freedom 

 from haziness, &c. It may be that the difference between the arc- 



