106 Dr. G. A. Liebig on the Electrostatic Force 



Only in one case (with hydrogen containing air) have I 

 observed fine stratifications in a high vacuum. 



If the tube contained oxygen (prepared from potassium 

 chlorate) the luminosity began at about 30 millim. ; the 

 spectrum was brightest at about 0*2 millim. The two bands 

 563-556 and 529-523 were specially conspicuous, and these 

 two could be recognized with greater exhaustion when the 

 tube was only faintly luminous. At high vacua the light 

 failed oftener than with the hydrogen-tube. 



With nitrogen I obtained in the small spectroscope the 

 same spectrum as with air. 



Helsingfors, May 26, 1885. 



XII. On the Electrostatic Force required to produce Sparks in 

 Air and other Gases. By G. A. Liebig, Ph.D., Johns 



Hopkins University*. 



[Plate II.] 



THE following experiments were undertaken at the sug- 

 gestion of Prof. Rowland, and the results submitted from 

 time to time to his inspection. 



Historical. 



Determinations of the difference of potential required to 

 produce sparks in air appear to be numerous. But the earlier 

 measurements, such as those made by Voltaf, HiessJ, and 

 Gaugain §, in which the difference of potential was determined 

 in arbitrary units by means of electroscopes, unit jars, &c, 

 have little more than a historic value. It was, however, 

 noticed by these observers that the length of spark increased 

 more rapidly than the charge ; and Van Oettingen || deduced 

 an empirical formula, 



q — clog (1 + el), 



in which c and e are constants, I is the length of spark, and 

 q the charge, to express this relation. The first experiments 

 on the subject, giving results in absolute measure, were made 

 by Sir William Thomson^. In his investigation the sparks 

 were made to pass between two plates, one of which was flat, 

 the other slightly convex ; and the difference of potential 



* Communicated by the Author, 

 t Identita, p. 53. 

 % Pogg. Ann. xl. p. 333 (1837). 

 § Ann. de Chim. et de Phys. viii. p. 108 (1866). 

 || Pogg. ^4ww. Jubelbd., p. 275 (1874). 

 5T ' Electrostatics and Magnetism.' 



