17 Mr. W. Spottiswoode. On Effects produced by [Feb. 19,. 



one or the other is more easily vaporised ; and when it is lengthened 

 they disappear. 



From this we may conelnde (1) that the discharge which we have 

 been examining, is a real flame with metallic particles passing between 

 the terminals in a solid condition ; and (2) that in general the tempe- 

 rature is comparatively low, i.e., that it is insufficient to cause any con- 

 siderable vaporisation. This is notably the case when the arc is long, 

 and when the matter thrown off from the terminals has sufficient time 

 in its passage to cool. 



The spark was then tried between carbon terminals in atmospheres 

 of hydrogen and of carbonic acid. In none of them did the spectrum 

 show any gas lines ; but with hydrogen there were faint traces of the 

 hydrocarbon group in the green. In this respect the spark differs 

 from the discharge direct from the machine, inasmuch as the latter 

 gives some of the hydrogen lines in hydrogen and carbon lines in 

 carbonic acid. 



When magnesium terminals were used in an atmosphere of 

 hydrogen, the yellow sodium lines, the blue and green magnesium 

 lines, and the red line of hydrogen were visible near the terminals, 

 with a continuous background.* 



When the spark was discharged in a magnetic field, known pheno- 

 mena were reproduced, but owing to the thickness and mass of the 

 flame and the extraordinary strength of the magnetic field, they were 

 exhibited in a state of great splendour, f 



When the spark passed in an equatoreal direction the whole flame 

 was spread out in an equatoreal plane, in which heated masses might 

 be seen revolving in one direction or in the other in the neighbourhood 

 of each of the magnetic poles. To give some idea of the actual 

 appearance, it may be mentioned, that a symmetrical spindle-shaped 

 discharge, 15 millims. in length and 3 millims. in thickness, was 

 spread out by the magnet into a disk of about 15 millims. in diameter 

 and 1 millim. in thickness. When the spark passed in an axial 

 direction, or when the poles themselves were made the terminals, the 

 phenomena described in my paper " On an Experiment in Electro- 

 Magnetic Hotation" (" Proc. Roy. Soc," vol. xxiv, p, 403) were 

 reproduced. 



Whatever was the direction of the spark, the resistance due to the 

 magnetic field was such as to extinguish the discharge, provided that 



* When the same discharge was effected in a magnetic field, the only change ob- 

 served was that the fines became slightly fainter. There was some reason to think 

 that, at the moment of making the magnet current, the lines became decidedly 

 brighter, and that they lost their brilliancy if that current was maintained. But 

 this, perhaps, requires further examination. 



f Through the kindness of the authorities of the Eoyal Institution, Faraday's 

 great electro-magnet, excited by the Siemens machine, was placed at my disposal. 



