Prof. E. Edlund on the Galvanic Luminous Arc. 411 



not lose heat without a corresponding amount of condensation, 

 the return into water of a portion of the working steam equiva- 

 lent to the work done seems to be an inevitable conclusion. 

 Hirn's later experiments on the working of large steam-engines, 

 described in his " Exposition Analytique," 1865, seem to prove 

 this relation ; and it is now supposed that the anomalous results 

 of former experiments are attributable chiefly to the presence of 

 water in the steam. This point has been fairly proved by the 

 use of superheated steam in Hira's more recent experiments ; 

 and a fresh series of experiments which I have been recently 

 making with a different method, which 1 believe to be novel, 

 lead to the same conclusion in a manner which does not seem 

 to admit of further doubt on the subject. I shall probably ere 

 long be requesting of you the favour to allow a brief statement 

 of these experiments to appear in the Philosophical Magazine ; 

 meanwhile the insertion of the present remarks would much 

 oblige me. 



I remain^ Gentlemen, 



Your obedient Servant, 

 Palermo, March 19, 186*8. Joseph Gill. 



LIT. On the least Electromotive Force by which a Galvanic Lu- 

 minous Arc can be produced. By E. Edlund*. 



HEN the conduction between the poles of a powerful 

 voltaic battery is interrupted, and a luminous arc 

 formed at the break, the intensity of the current is known to be 

 considerably less than when the conduction is unbroken, and no 

 light exists. It has hitherto been assumed that the great dimi- 

 nution in the intensity which here ensues arises from a great re- 

 sistance which the luminous arc offers to the transmission of the 

 current. It has been found that this resistance increases with 

 the length of the arc. In a previous investigation f I have 

 proved experimentally that if an ordinary resistance be inter- 

 posed in the circuit which causes the same diminution as the 

 luminous arc alone, this consists of two parts, one of which is 

 quite independent of the length of the luminous arc, and is un- 

 changeable in magnitude so long as the intensity remains the 

 same; the other part increases proportionally to the length of 

 the luminous arc, provided that, when the length of the arc is 

 increased, so much of the interposed resistance is removed that 

 the intensity of the current is not thereby diminished. If the 



* Poggendorff's Annalen, No. 3 (1868), having been reail before the 

 Swedish Royal Academy of Sciences, December 11, 1S67. 



t Poggendortf's Annalen, No. 8, 1867. Phil. Mag. vol. xxxv. p. 103. 



