The Effect of Pressure upon Arc Spectra, 5G9 



witli, tho ciuantitiou of radium, which is dissolved intlie water. 

 So far there is no indication of the presence of a sufficient 

 amount of an emanating substance present in the water to 

 account for the etfects observed. The greater part of the 

 emanation must be supposed due to radioactive emanating 

 substances distributed near the source of the water. 



It is a pleasure to express my sincere thanks to Prof. 

 Thomson for tlie continual interest he has shown and help he 

 has i>iven durino- the course of this work. 



Cavendish Laboratory, July 1903. 



LXXII. Preliminary Note on the Effect of Pressure upon Arc 

 Spectra. By J. E. Petavel and R. S. Hutton"^. 



[Plate XXV.] 



Introduction. 



IN the course of an investigation on the effect of high 

 gaseous pressures upon the chemical reactions occurring 

 at the temperature of the electric furnace, it was found ad- 

 visable to make experiments with electrodes of different 

 materials ; the spectra as observed visually seeming to show 

 some points of interest, the brief investigation of the subject 

 herein described was underta^^en. The shift of the spectrum 

 lines caused by pressnre has already been carefully investi- 

 gated by Humphreys, Mohler, Jewell, and others f. A 

 higher dispersion than that available in our experiments 

 would be necessary to add anything further to this side of 

 the question, which, therefore, we do not propose to deal 

 with. In general, such results as are given and the explana- 

 tions proposed must be regarded as preliminary in character ; 

 but it is hoped that before long the work will be continued 

 in the Physical Laboratories of the Owens College. 



The apparatus consisted of a strong steel enclosure, suitable 

 for withstanding pressures np to 450 atmospheres, having an 

 internal diameter of 11 inches and a capacity of about 20 

 litres. The electrodes, which were much larger than is usual 

 for such work, varied in diameter from \ inch to 2 inches, 

 they were supported by feeding rods passing air-tight through 

 stuffing boxes, and capable of adjustment by a powerful screw 

 gear. Apart from the usual valves for admission or circula- 

 tion of gases under pressure, the apparatus is provided with 



* Communicated by Prof, Arthur Schuster, F.R.S. 



t Jewell, Astrophys. Journ.iii. p. 89 (1896) ; Humphreys and Mohler, 

 Astrophys. .Journ. iii. p. 114 (189(5); Mohler, Astrophys. Journ. iv. 

 p. 175 (1890) ; Humphreys, Aslroplivs. Journ. iv, p. 249 (189G), vi. 

 p. 169 (1897j. 



