17(5 'lemperature of the Crater of an Arc Light. [May 30, 



moderate vacuum, I found the radiation of the crater to be much 

 greater than at the atmospheric pressure. 



The temperature of the crater seemed very sensitive to any sudden 

 diminution of pressure in the gas. If the blow-off valve was suddenly 

 opened, the brilliancy of the crater fell so much that it became nearly 

 invisible. When the box was being exhausted by the air-pump, 

 although the temperature of the crater was rising as the vacuum 

 improved, yet at each stroke of the pump the eye could see a distinct 

 falling off of brilliancy in the image. 



It was thought that the diminution of brilliancy might be due to 

 smoke inside the box, but on looking through the window everything 

 was seen sharply denned, also the gas as it issued from the blow-off 

 was perfectly clear. The arc was also kept burning for some time 

 in the box at the atmospheric pressure, but the image remained quite 

 clear, and the inside of the box seemed quite free of smoke. 



From these experiments it would seem as if the temperature of the 

 crater, like that of a filament in an incandescent lamp, depends on how 

 much it is cooled by the surrounding atmosphere, and not on its 

 being the temperature at which the vapour of carbon has the same 

 pressure as the surrounding atmosphere. That carbon volatilises in 

 some form at comparatively low temperatures seems likely, from the 

 way in which the carbon of incandescent lamp filaments is transferred 

 to the glass. The pressure of the vapour of carbon in the arc may 

 consequently be very small, and further it would seem that the 

 supposition of high pressures in the solar photosphere, which has 

 been referred to in the beginning of this paper, is not borne out by 

 these experiments, and that carbon may exist there in the solid form 

 at very high temperatures although the pressures are comparatively 

 low. 



The experiments on high pressures were conducted on several 

 occasions. On the last occasion, in addition to repeated former 

 experiments, the experiments on reduced pressures were performed, 

 and I then had the great advantage of the presence, advice, and 

 assistance of ray friends Professor Minchin and Professor Gr. F. 

 Fitzgerald. The later series of experiments entirely confirmed my 

 former ones. 



