Occurrence of Enhanced Lines in the Arc. 287 



is reduced, with consequent rise o£ temperature of the second 

 wire. Such an effect would be absent when the second wire 

 conveys only a very small current. From what has been 

 already said, it is clear that the distances of" the wires apart 

 involved in this consideration are somewhat greater than 

 those considered in the discussion of the curves E to H 

 (pp. 281-2). 



The experimental work detailed herein was carried out in 

 the Physical Laboratory of the South Metropolitan Gas 

 Company, and the author desires to express his gratitude to 

 Dr. Charles Carpenter, C.B.E., M.I.C.E., for the provision 

 of the facilities necessary for the prosecution of the work. 



Physical Laboratory, 



South Metropolitan Gas Co., 



709 Old Kent Road, S.E. 



Aug. 15, 1921. 



XXVIII. Observations and Experiments on the Occurrence of 

 Spark Lines (Enhanced Lines) in the Arc. — Part I. 

 Lead and Tin. By G. A. Hemsalech and A. de 

 Gramont *. 



[Plates ni.-V.] 



Contents. 



§ 1. Introduction. 



§ 2. A convenient arrangement for obtaining the arc spectra of 

 metals having low melting-points. 



§ 3. Spectroscopic methods employed. 



§ 4. Preliminary observations. 



§ 5. Effect of a drop of liquid placed between the electrodes. 



§ 6. Effects of various gases. 



§ 7. Arc in liquid air. 



§ 8. Nomenclature employed in this research. 



§ 9. Results of observations on the arc spectra of Lead and Tin. 

 § 10. Explanation of Plates. 

 §11. Summary of results. 

 § 12. Concluding remarks. 



§1. Introduction. 



IN the course of his classical series of laboratory experi- 

 ments Sir Norman Lockyer observed that certain 

 spectrum lines which were absent from the flame and 

 relatively feeble in the arc, became bright relative to other 

 lines when a powerful condenser spark was used as the source 

 of light. The name " enhanced lines " was given to this 

 particular type of light radiations, and their relative 

 strengthening on passing from the flame to the arc and 



* Communicated bv the Authors. 



