19 
Professor Leslie on Electrical Theories. 
natural, and so agreeable to the primary information of the 
senses, that scarce a doubt has ever been entertained of its 
reality. But what was presumed to be the electric fluid, is on- 
ly air endued with certain properties. The existence of such a 
fluid is therefore illusory ; it is unnecessary, and inconsistent 
even with mechanical principles. 
If a certain change in the elementary arrangement of a body 
produce an emission of light, an opposite change may occasion 
an absorption . And, when instances of that kind occur, they 
will completely escape our observation ; since the eye can judge 
of those rays only Avhich enter itself. Nor will the brilliancy of 
a neighbouring body be affected ; for, as the action in every 
case diminishes with the distance, the particles of light will be 
imbibed from the immediately contiguous matter, which will 
then be disposed to attract from the next adjacent portion, and 
thus, as the effects extend from the centre, they will gradually 
be attenuated, and become insensible. On the contrary, when 
there is an emission, a vast force is exerted at first, which com- 
municates to the particles an illimited progressive motion. 
On every hypothesis, it will readily be admitted, that the 
electricity of the cushion is the reverse of that of the conductor; 
for what is attracted by the one is at the same time repelled by 
the other. If, therefore, a body in contact with the one emits 
light, we may infer, from the contrast of their properties, that a 
body connected with the other is disposed to absorb it. The 
aerial streams between two adjacent bodies will consequently be 
affected oppositely in regard to light, and, to discriminate them, 
we have only to ascertain the direction of that which is lucid. 
The most certain and easy method is to trace the curve of their 
motion. The arguments commonly adduced to prove the exist- 
ence of an electric fluid, may with equal propriety be employed 
to demonstrate the materiality of gravitation and magnetism. 
Nay, it might by the same reasoning, as in the infancy of 
science, be urged, that a moving body is actuated by a certain 
vital principle. 
The denominations of positive and negative electricity are 
purposely omitted in this discourse, because they are founded 
on the hypothesis of an electric fluid. Let A and B (PI. I. Fig. 5.) 
be two small bodies possessing opposite electricities. It is ob- 
