Mr. Liming on the primary Forces of Electricity. 49 



of mechanical pressure which air exercises." In the valuable 

 magazine of facts to which I have found occasion so frequently 

 to refer, there is enough said to carry with it a conviction 

 that Mr. Harris is by no means satisfied of the truth of the 

 supposition ; indeed he has in one place actually proved that 

 the resistance to electrical discharges does 7iot vary as the 

 atmospheric pressure ; it would appear however that he is 

 unconscious of his achievement ; for in other places we find 

 him insisting on the opposite conclusion as the legitimate in- 

 duction of his experiments generally. 



50. The proof to which I allude is furnished by a series of 

 experiments, in which the temperature of a given quantity oj 

 air inclosed in a receiver was made to vary between 50 and 

 300 degrees of Fahrenheit, without " in the least " affecting 

 the discharging distances of the electrical accumulations ; 

 notwithstanding the atmospheric pressure at different times 

 of course differed enormously*. In these experiments, as in 

 all the preceding, the discharging distances were as the quan- 

 tities of compensating air, and therefore by our principles as 

 the quantities of free electricity ; that is to say, as the abstract- 

 ing force minus the retarding force of the minor electrical 

 attraction. 



51. I have made many experiments on the discharging di- 

 stances of free electricity in receivers filled with rarified air, 

 without finding anything which offers the least evidence of 

 electricity being retained on the surface of bodies by reason of 

 atmospheric pressure ; but on the contrary every evidence 

 which the investigation was capable of affording has been ob- 

 tained in favour of the principles generally on which this theory 

 is based. For instance, let the density of the air, or its press- 

 ure, have been what it may, the discharging distances have 

 invariably increased in the simple ratio of the quantities, until 

 the sides of the receiver and the external air have by their 

 comparative proximity interfered with the ratio by assuming 

 part of the compensation of the charge. 



In the following experiments, which are adduced as ex- 

 amples of such an interference, the quantities of electricity 

 were estimated by the unit jar; the discharges were made 

 between two brass balls Ifths of an inch in diameter, placed 

 in the axis of a receiver, more or less exhausted, about 4^ 

 inches in diameter and IS inches high ; and the balls commu- 

 nicated, respectively, with the opposite conductors of a Ley- 

 den jar exposing about two square feet of coating. 



* Phil. Trans. 1834, p. 229, 230. 

 PMl. Mag, S. 3. Vol. 13. No. 79. July 183S. E 



