278 Dr. Harems Electrical Machine, &fc. 



ducting air. A small ball, negatively electrified, can only be pro- 

 ductive of a diffuse attraction for the electricity in the atmospheric 

 medium around it ; so that it has less ability to create any penetra- 

 ting power, than when acting upon the electricity in a comparatively 

 large globular conductor, as in the preceding illustration. Hence, 

 when the knob is on the negative pole, it may be productive of a 

 luminous appearance in its immediate vicinity, where the electric 

 matter, converging from the adjoining space, becomes sufficiently in- 

 tense to be productive of light ; but it does not produce the striking 

 appearance of the luminous brush. 



As, agreeably to Du Fay's theory, the knob, whether vitreously 

 or resinously electrified, is surcharged with an electric fluid, the pro- 

 jectile power ought to be as great in the one case as in the other ; 

 and the long spark and the brush should be producible in either case. 



On some Inferences from the Phenomena of the Electric Sparlc, in 

 a recent work on Heat and Electricity. 



In his valuable work on heat and electricity. Dr. Thomson states 

 that if a long spark be taken between two knobs, as when severally 

 attached to the positive and negative conductors of the electrical ma- 

 chine ; the portion of the spark near the positive knob exhibits all 

 the characters of positive electricity, while the remaining portion 

 proceeding from the other knob displays all the characters of nega- 

 tive electricity. Although the learned author does not state what 

 differences there are between the different portions of the spark, 

 and wherefore, if any exist, he can, without a petitio principii, as- 

 sume that they are such as to justify his conclusion ; he proceeds 

 to allege that there can be no doubt that every spark consists of 

 two electricities ; and that these, issuing severally from their respect- 

 ive knobs, terminate their career by uniting at the non-luminous por- 

 tion of the spark, which is at a distance from the negative knob of 

 about one third of the interval. Upon these grounds he infers that 

 the positive electricity occupies two thirds of the length of the spark, 

 the negative one third. 



I presume that, agreeably to the theory which supposes the exist- 

 ence of two fluids, when the equilibrium between oppositely excited 

 surfaces is restored by a discharge, whether in the form of a spark or 

 otherwise, there must be two jets or currents passing each other ; 

 the one conveying as much of the resinous as the other does of the 

 vitreous electricity. Of course no part of a spark can be more nega- 



