278 Dr. Hare’s Electrical Machine, &c. 
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 Pees the striking 
appearance of the luminous brush. 
As, agreeably to Du Fay’s theory, the a 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 Spark, i 
a recent work on Heat and Electricity. 
Iv 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 i is restored by a discharge, whether in the form of a spark or 
‘otherwise there must be two jets or currents passing each other; 
myeying as much of the resinous as the other does of the 
lectricity. Of course no part of a spark can be more nega- 
