230 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



of fallacy appears to consist in a neglect of a very important element, — the density of 

 the air immediately operated on, — and which has great influence in the restraining 

 of electrical discharges (44.). By means of the arrangement above described (43.), 

 this source of fallacy is altogether avoided, and we are enabled to experiment on a 

 constant volume of air of variable temperature. 



(m.) The experiment being disposed as above stated (44.), an accumulation was 

 effected sufficient to discharge through a certain interval of air of a given temperature, 

 and whose volume was fixed by closing the cock r, fig. 16. This being ascertained, the 

 temperature was varied from between 50 to 300 degrees of Fahrenheit, but without 

 in the least affecting the result ; the discharge invariably occurred when the same 

 quantity was accumulated. The influence of heat was therefore evidently not in any 

 way opposed to the restraining power of the air. 



(n.) The heated air was now permitted to expand, by opening the cock at r, and 

 allowing an escape through the long gage g, from under the surface of the mercury 

 in the cistern w : when the full expansion had taken place, the cock was again 

 closed. The thermometer within now stood at about 280 degrees. This preparation 

 being accomplished, the quantity requisite to cause a discharge between the balls c o 

 was again determined ; but although greatly reduced, it was found to remain the 

 same through each succeeding decrease of temperature, as the whole gradually 

 acquired the temperature of the room. When this was attained, the cock at r was 

 again opened, in order to admit of the ascent of the mercury in the gage, and by 

 which the density of the air in the receiver could be sufficiently well estimated. The 

 comparative accumulation, as in the preceding cases (44.), was then found to be as 

 the diminished density directly, or nearly so. 



49. These experiments on the power of heated air to restrain electrical discharges, 

 were varied in the following way : A portion of the air within the receiver R, fig. 17, 

 was first withdrawn, so as to raise the mercury in the long gage g about five or six 

 inches ; a given accumulation was then effected, sufficient to produce a discharge be- 

 tween the opposed spheres c c\ The receiver was now heated as before, and the 

 descent of the mercury in the gage observed. By this method the actual tension of 

 the air within could be estimated, whilst the expansive force on the plates h K ter- 

 minating the receiver, was efficiently resisted by the atmospheric pressure from with- 

 out, so that the plates did not require further support. The results were the same as 

 those before arrived at. The insulating power of the air was found to be quite inde- 

 pendent of its temperature, and to depend only on the density. 



50. We may conclude from these experiments, — 1°. That heated air is not, as fre- 

 quently stated, a conductor of electricity, and that heat does not facilitate electrical 

 transmission through air in any other way than by diminishing its density ; — 2°. Sup- 

 posing heat to be material, it is a non-conductor of electricity ; because the incor- 

 poration of a conducting with a non-conducting substance is found to impair the 

 insulating power of the latter, as in the case of air charged with free vapour; 



