the Occurrence of Enhanced Lines in the Arc. 297 



named metal as electrodes in this gas a stable arc becomes 

 established. This fact of course implies a state of high con- 

 ductivity of the vapours — a condition which, as our previous 

 results had shown, is adverse to the emission of spark lines. 



§ 7. Arc in liquid air. 



Our preliminary experiments had taught us that spark 

 lines are brought out when the electrodes, especially the 

 cathode, remain relatively cool during arcing, as for example 

 when a graphite rod is used facing a metal pole. It seemed 

 therefore desirable to make some observations on the spectrum 

 of the arc with the electrodes cooled down to the temperature 

 of liquid air — namely, to about — 190° C. It was obviously 

 not possible to apply the method of the liquid drop' to this 

 case ; therefore the electrodes were mounted inside a double- 

 walled, glass vessel and completely immersed in liquid air. 

 No stable arc was obtained ; only short arc flashes, each one 

 of which was accompanied by the evolution of numerous air 

 bubbles caused by the boiling of the liquid around the path 

 of the discharge. In the spectrum of these luminous flashes 

 the spark lines or so-called high temperature lines were 

 strongly developed, and generally remained visible till the 

 end of the flash. The flame lines, on the other hand, were 

 relatively feeble. The arc lines showed a normal develop- 

 ment as in an ordinary arc. Besides confirming our 

 previous conclusions with regard to the effect of cooling 

 of the vapour on the appearance of the spark lines in the arc, 

 Vhese results further emphasize the fact that the presence of 

 hydrogen is not essential for their emission. 



§ 8. Nomenclature employed in this research. 



In connexion with the various modes of obtaining the arc 

 spectra of metal vapours as detailed in §§ 2-7, the following 

 denominations will henceforth be adopted in order to secure 

 precision and avoid repeated explanations. 



1. Carbon or graphite arc. — Arc formed between a metal 



and a carbon or graphite rod. 



2. Direct arc* — Arc passing between two metal electrodes. 



3. Ordinary arc. — A carbon, graphite, or direct arc 



burning steadily and continuously. Observations, 

 either visual or photographic, are begun only after 

 the arc has been started and are stopped before the 

 arc is extinguished. 



* A. de Gramont, loc. cit. 



