Conductivity of Gases. 315 



and that in the spectrum of the arc the corresponding 

 metallic lines appear sharply. 



According to Casselmann* the light-arc is of different 

 length with different metals, and is so much the longer the 

 more volatile these are. 



He arranges the metals in the following order: — Potassium, 

 sodium, zinc, mercury, iron, tin, lead, antimony, bismuth, 

 copper, silver, gold, platinum. So that potassium yields the 

 longest arc, and platinum the shortest. 



As already remarked, there are essentially two factors to be 

 taken into account — the true conductivity of the vapours, and 

 the quantity in which they are formed ; since further, the 

 conductivity depends in a high degree upon the temperature, 

 it is not to be wondered that HittorPs experiments in the 

 Bunsen flame, and Casselmann's in the arc-light, have given 

 different results. 



§ 8. It is less easy to understand the conditions which 

 obtain in the carbon light. That here also metallic vapours, 

 which are introduced into the arc, increase the conductivity, 

 is of course obvious from the preceding results, and it is not 

 even necessary to refer to the data of Casselmann. Hence 

 we obtain a longer arc between carbons which have been 

 saturated with metallic salts than between carbons as they 

 are found in commerce. 



But that metallic compounds are abundantly present also 

 in the carbons which are now so largely used for electric 

 lighting may be easily shown. As is well known, the 

 spectrum of the electric arc shows a multitude of bright 

 lines, which are particularly distinct at the moment of inter- 

 rupting the current, when the continuous spectrum of the 

 white-hot particles of carbon has faded. 



The sodium-line is especially brilliant ; besides which we 

 always observe the lines of calcium, iron, and magnesium. 

 The result is always the same, of whatever manufacture the 

 carbons may be ; and it may perhaps be of interest to compare 

 different opinions as to the preparation of good carbons. 

 According to Carre t, good carbons may be obtained by 

 compressing an intimate mixture of lampblack and powdered 

 coal, and strongly igniting after addition of iron, antimony^ 

 or tin, or by boiling the carbons for a long time in a solution 

 of metallic salts. Archereau and Sandain recommend a 

 similar method. Jacquelain \ proceeds in an entirely different 

 way, since he endeavours to remove all mineral constituents 



* Pogg. Ann. lxiii. p. 576 (1844). 



t Comptes Hendusy lxxxiv. p. 346 (1877). 



% Pogg. Ann. Jubelband, ccccxl. (1874). 



