1 6 Professor Leslie on Electrical Theories. 
If we examine the subject with attention, we shall be con- 
vinced, that the only information which the eye ever conveys, 
is limited entirely to the quantity, the quality, and the direction 
C?f the rays which enter it. It is from the sense of feeling alone, 
which is diffused over the whole surface of the body, that we 
derive our ideas of figure and extent, of motion and force, which 
are the foundation of all our knowledge of matter. But from 
the constant habit of comparing the impressions of touch with 
certain indications of sight, we learn to infer the one species of 
sensation from the other. Nay, in most cases, the information 
communicated by vision serves only to suggest the other ideas. 
However, if a substance emits light profusely, the idea excited 
by the means of sight will become prominent, and occupy a 
principal share in the complex notion which we form. Such is 
the case when the substance is solid and unvaried ; but, if it 
be a fluid of great volubility, the faint and imperfect indica- 
tions of the sense of feeling will be lost in the powerful impres- 
sions of its brightness. For instance, we know that heated tal- 
low discharges inflammable gas, whose union, at a high tempe- 
rature, with pure air, forms another compound, to-wit, steam. 
When a candle is lighted, this combination immediately takes 
place, and the two kinds of air, suffering a considerable change 
of temperature, become disposed , by a certain property, which 
is foreign to the present inquiry, to discharge a large portion of 
the luminous matter which they previously contained. In this 
case, the idea of brightness predominates ; we forget the prior 
condition of the aerial substances ; and early prejudices may 
perhaps suggest the independent existence of Jlame. But every 
body in nature can be rendered luminous by heat, by compres- 
sion, or by attrition, and by the operation of all those causes 
which produce certain alterations of texture; and the intensity 
of its brightness is ever proportioned to the quantity of change 
induced upon it * *. 
or a shock. But if one end of a silver thread , consisting of silk , coated with the ihin- 
nest film of metal , and two or three yards in length , be tied to the neck of a large in- 
sulated brass ball , at three or four inches distance , and the other end held hi the 
hand , such spark from the machine will be accompanied by a lucid glow over the 
whole extent of the thread. 
* Since this assertion was made , air, and the different gases , water and oil \ 
have been made , by sudden compression , to emit flashes of light variously coloured l 
