﻿Fluorescence Spectrum of Iodine Vapour. 329 



Results. 



(1) The insulating liquids do not differ very much from 

 one another in power of resisting discharge, the strength 

 lying between the limits 110 volts per micron and 70 volts 

 per micron, though some liquids seem to have slightly less 

 strength. The range of voltage here spoken of is between 25 

 and 400. 



(2) Some liquids have a peculiar variation in potential 

 gradient tor distances between Ijx and 2fi. Castor oil, olive 

 oil, and paraffin are cases in point. 



(3) All insulating liquids have greater strength than air 

 for P.D. greater than 3(K) volts ; for less P.D. liquids have 

 much less strength than air. 



(4) The results for a homologous series of hvdrocarbons do 

 not indicate any simple connexion between composition and 

 power of resisting discharge ; the differences found are as 

 likely to be due to accidental imparities as to specific qualities 

 of the liquids. 



But these simple snb-tances have specially great dielectric 

 strength. It may be that simplicity of composition makes for 

 strength. 



(5) Whereas in the case of gases there is always a hori- 

 zontal portion in the curve, in liquids there is no such distinct 

 break in the curve b 'tween 25 and 400 volts, unless the 

 region at about 60 volts in castor oil, olive oil, and paraffin 

 is of this nature. 



(6) No simple connexion can be observed between power of 

 resisting discharge and specific inductive capacity, in liquids. 



(7) The temperature throughout was from 15° C to 18° 0. 



XXXIX. The Fluorescence, Magnetic Rotation, and Tempera- 

 ture Emission Spectra of Iodine Vapour. By R. W. Wood, 

 Professor of Experimental Physics, Johns Hopkins 

 University*. 



[Plate IV.] 



THE fluorescence of iodine vapour has been observed by 

 Wiedemann and Schmidt, and also by Konen, but the 

 luminosity appeared to be too feeble to allow a study, either 

 visual or photographic, to be nw'e of its spectrum. By 

 studying the conditions under which the fluorescence manifests 

 itself to the best advantage, it has been found possible to 

 photograph its spectrum with a large three-prism spectro- 

 graph, with its slit narrowed to such a degree that the 



* Communicated by the Author. 



