ELECTRICAL DISCHARGE THROUGH AIR OF VARIABLE TEMPERATURE. 229 



coating, fig. 17, given quantities of electricity were accumulated, and the distances at 

 which the discharge occurred in air varying in density observed, this series of experi- 

 ments led to the following results : 



(Jc.) 1st, The respective quantities requisite to pass a given interval, c c\ varied in a 

 simple ratio of the density of the air. When the density was one half as great, the 

 discharge occurred with one half the quantity accumulated ; that is to say, with one 

 fourth of the intensity or free action (16. 20.). 



(/.) 2ndly, The distance c c' through which a given accumulation could discharge, 

 was found to be in an inverse simple ratio of the density of the air, the intensity or 

 free action being supposed constant. In air of one half the density, the discharge 

 occurred at twice the distance. 



45. These results are in complete accordance with the conclusion already derived 

 (37.)j since the attractive force between the points of discharge c c' was, 1°, varied by 

 varying the whole quantity accumulajted ; 2°, by varying the distance : the force, 

 therefore, was in each case the same (20. 67-), that is, as the square of the density 

 of the air directly. 



46. By diminishing the density, and increasing the distance between the points of 

 discharge, we may very completely represent the beautiful phenomena of summer 

 lightning : the electrical explosion approaches nearer and nearer to the state of a 

 diffuse luminous flash without noise : and this also happens when the distance between 

 the discharging points is the same, the quantity accumulated becoming continually 

 reduced. We may hence infer, that in atmospheric discharges between clouds op- 

 posed to each other in air greatly rarefied, either by heat or by diminished pressure 

 (44. /., 50.), the electrical accumulation never proceeds beyond a certain limit ; so 

 that discharges in diffuse flashes, without noise, repeatedly occur, whilst the exciting 

 cause of the electrical accumulation continues to operate. 



47. The resistance of a column of non-conducting matter, such as air, to the passage 

 of an electrical discharge, appears by the foregoing ^ults (43.) to produce a some- 

 what different result to that of conducting bodies ; th^ftsistance in the former arising 

 solely from the pressure of non-conducting particles, by which the whole accumula- 

 tion is restrained within given limits. Now when the attractive forces are sufficiently 

 great to remove the atmospheric column interposed between the points of discharge, 

 the accumulated electricity escapes in a dense form between those points, without any 

 regard to the distance traversed ; and without any intermediate operation on the force 

 of the electrical current, as in the case of electricity passing through an interposed 

 circuit of metal of greater or less extent (40.). 



48. I endeavoured to find, by varying the temperature of a given volume of air 

 forcibly retained within the receiver R, figs. 16, 18, so as to prevent expansion, 

 whether the influence of heat was such as to impair its insulating property. It may 

 be here remarked, that the numerous experiments hitherto instituted, in order to show 

 the conducting power of heated air, are by no means conclusive. The great source 



MDCCCXXXIV. 2 H 



