318 Mr. R. H. M. Bosanquet on Electromagnets. 



light disappeared each time. It follows from this, as well as 

 from former observations of Hittorf* and W. Siemens f on 

 the luminosity of flames, that up to the temperature of 

 melting iridium, gases possess no perceptible emissive power, 

 if they are not concerned in chemical changes or in electric 

 discharges. The cessation of luminosity at the white heat of 

 the electrodes may perhaps be explained by the hot metallic 

 vapours taking over the conduction, so that only a small 

 fraction of the current goes through the hydrogen or 

 nitrogen, which is not sufficient to bring it to luminosity. 

 The conditions under which metallic vapours emit light may 

 be quite different from those which hold good for ordinary 

 gases. The investigations of Schuster \ have shown, at least, 

 that mercury vapour in a vacuum tube, from which every trace 

 of other gas has been expelled, presents essentially different 

 and, in fact, simpler phenomena, so that the electric discharge 

 takes place without glow-light, dark space, or stratifications. 

 Whether Schuster's explanation is correct, that the reason is 

 to be found in the simpler constitution of mercury vapour, or 

 whether metallic vapours generally do not show this simpler 

 mode of discharge, seems to be worthy of farther investigation. 

 Since I have myself always seen an unstratified coherent 

 mass of light filling the space between the electrodes in an 

 arc of 3 centim. in length, the second possibility appears to 

 me the more probable. 



Physical Institute, 

 University of Strassburg. 



XXXV. Electromagnets. — IV . Cast Iron, Charcoal Iron, and 

 Malleable Cast Iron. By R. H. M. Bosanquet, St. John's 

 College, Oxford. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 rpHE following measures of permeabilities have been made 

 -A- on a number of rings of cast iron, charcoal iron, and 

 malleable cast iron. The magnetizing forces used varied in all 

 cases from small forces to forces large enough to raise the 

 metal to a condition but little removed from saturation. I do 

 not include at present any comparisons with formulae, as the 

 facts are perhaps best kept separate in the first instance. 

 These measures complete all the experiments on rings which 

 I have contemplated so far. 



It may be convenient to summarize shortly some chief 

 points of the results of all the experiments on rings. 



* Wied. Ann. vii. p. 587 (1879). f Ibid, xviii. p. 311 (1883). 



X Proc. Roy. Soc. xxxvii. p. 317 (1884). 



