244 Mr. Gr. A. Hemsalech : Excitation of Spectra of Carhon r 



have incidentally brought out some very interesting par- 

 ticulars concerning the formation of the electric arc which 

 point to the existence of a critical temperature of the elec- 

 trodes for a given strength of electric field. 



The present paper will deal only with the luminous and 

 spectroscopic effects which are caused by the passage of 

 therm electronic currents through the vapours in the vicinity 

 of a single plate of graphite. 



The experiments involved in the whole of this investiga- 

 tion have occupied the greater part of my time during the 

 past two years, and I take this opportunity of placing on 

 record my deep sense of gratitude to the Council of the 

 Royal Society and to the Committee of the Privy Council 

 for Scientific and Industrial Research, who by their generous 

 financial assistance have rendered these researches possible. 



I have also much pleasure in expressing my heartiest 

 thanks to Sir Ernest Rutherford for the very cordial hospi- 

 tality which he has extended to me in his laboratory and for 

 the facilities of all kinds which he has most kindly placed in 

 my way for carrying out these experiments. 



§ 2. Experimental Methods. 



When planning the experiments for these extensive re- 

 searches special attention was paid, from the outset, to 

 arranging them upon such a scale as to render their exe- 

 cution both possible and convenient for unassisted working. 

 This applied in particular to the size of graphite plates to 

 be adopted. Since several hundreds of experiments were 

 necessitated in the course of this investigation, the use of 

 large-sized plates, involving the running of heavy machinery 

 for producing the necessary heating current, would have 

 entailed considerable expense, both in material and labour, 

 not to mention the high consumption of electrical energy. 

 Accordingly a long series of trials was first made with 

 graphite plates of various dimensions and, as a result of this 

 preliminary work, a plate of small size was finally adopted 

 which could be raised to the requisite maximum temperature 

 by relatively feeble currents, such as can safely be taken for 

 a short time from an ordinary battery of accumulators. One 

 great advantage of small scale experiments is that their 

 repetition or extension in other laboratories is greatly 

 facilitated. 



The plates used in these experiments are made of Acheson 

 graphite. They are 10'0 mm. wide, 0*95 mm. thick, and 

 about 37 mm. long. Their ohmic resistance at ordinary 



