Professor Leslie on Electrical Theories . 
15 ' 
number of particles will, in a given time, come into contact with 
the point. Accordingly, electricity is never in the least corn- 
municated through a perfect vacuum. 
It is evident that the investigation which we have given will 
apply equally, whether the acute metallic body be originally 
electrified, or only be brought into the neighbourhood of an elec- 
trified substance, and whether its electricity be similar to that of 
the prime conductor, or similar to that of the rubber. But the 
form and quantity of the light exhibited on the points are diffe- 
rent in these cases, and it is an object of curiosity, at least, to 
inquire into the cause of this diversity of appearance. — This 
will require some discussion. 
The solar rays are distinguished from every other kind of 
matter by their passing through transparent substances, and by 
their suffering refraction and reflexion. But, in every case, the 
medium of vision must obviously possess these properties; and, 
therefore, all the various kinds of light are radically the same* *. 
Nay, it is highly probable that the lucid particles which bodies 
in certain circumstances emit, are all derived originally from the 
sun: For the solar rays are partly absorbed in their passage 
through pellucid substances; and in such as are opaque, a still 
greater portion of them is lost at the surface. A quantity of lu- 
minous matter may thus be accumulated and detained in bodies, 
and whenever they are disposed , by the change of structure or 
any other cause, to make a partial discharge, the emission will be 
attended with a degree of brilliancy. There is a distinction,, 
however, which may be remarked : The sun-beams consist of a 
due mixture of the coloured rays, and hence are bright white ^ 
but the ingredients of the other kinds of light are extricated in 
very different proportions, and from tliis diversity result ail the 
various tints "f*. 
Or all homogeneal light in passing through the same medium suffers the 
same degree of refraction. Therefore light is the same substance, from whatever 
source it proceeds. We may state also , that every species of light must have the 
same, or nearly the same celerity , else it would not enable our eyes , in their present 
structure , to perform the f unction of distinct vision. 
*1* Every substance whatever , while in a state of highly electrical excitement , 
wtll emit light , though this may not be always visible , since a certain duration and 
intensity of lustre are necessary to leave an impression upon our visual organs. 
Hence a metallic wire does not appear to- glow during the transmission of a spark 
