Prof. G. Wiedemann on Molecular Magnetism. 223 



deal with a microphonic action alone. And, indeed, in the 

 selenium cells I never could observe this fatigue. The sensi- 

 tiveness of the last-described piece is now extremely lowered. 



And thus we see that there are two different effects which 

 light may have on selenium and on selenium cells. The one 

 is more of a chemical, the other more of a physical character. 

 The one is a changing of modification, and is not essential to 

 the efficiency of the selenium photophone. In this latter we 

 have to deal essentially with a heating effect, changing volume 

 and contact — in brief, with a microphonic action. 



Therefore, as to the photophonic efficiency of selenium, 

 I see no reason to separate it from all other bodies ; and I no 

 longer believe that there is any prospect of finding an un- 

 known power or a new relation of forces in this substance. 



The above experiments were performed in Prof. Guthrie's 

 Physical Laboratory at the Science Schools, South Kensington. 

 The permission granted me, at his request, by the Department 

 of Science and Art to carry out my ideas experimentally is only 

 one of the many acts of kindness and courtesy which I have 

 received from scientific men and Societies during my sojourn 

 in England. 



Physical Laboratory, South Kensington, 

 June 1881. 



XXVII. Remarks on Prof. Hughes's Papers on Molecular 

 Magnetism. By Prof. G. Wiedemann*. 



IN the i Proceedings of the Royal Society of London,' 

 vol. xxxi. p. 532, vol. xxxii. pp. 25, 213 (1881), Prof. 

 Hughes has published a series of papers on the relations be- 

 tween magnetism and torsion, upon which I beg leave to make 

 the following remarks: — 



In his first paper, Prof. Hughes has quoted from my work 

 on Galvanism and Electromagnetism (1st ed. vol. ii. p. 438, 

 instead of p. 447 as cited) an experiment of mine, by which I 

 had shown that an iron wire through which a current flows is 

 magnetized by torsion. He says that Sir William Thomson 

 had attributed this magnetization to a spiral arrangement of 

 the current in the twisted wire. 



Now, in the passage above referred to, I state distinctly 

 that, if an iron wire is twisted during or after. the passage of 

 a current, it becomes magnetic. In the latter case there is no 

 longer any current in the wire. I have observed this magne- 

 tization directly by the deflection of a magnetic mirror, and 

 determined its direction. In accordance with many other 

 observations I have fully treated the rotations of the magnetic 

 * Commuuicated by the Author. 



