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the precise meaning of those terms which relate both to certain 
physical conditions of matter, and to the mental impressions 
which arise from them, namely; Heat, Light, and Sound. 
Heat was 200 years since very accurately defined by one of our 
ablest philosophers, as well as most precise and logical writers, 
John Locke, he writes: — “ Heat is a very brisk agitation of 
the insensible parts of the object, which produces in us that 
sensation from whence we denominate the object hot ; so what 
in our sensation is heat, in the object is nothing but motion.” 
It would be perhaps still more precise to say, “ heat arises 
from,” &c., in place of “ heat is,” &c., because the latter part of 
the definition states heat to be not the motion, but the perception 
of it. 
24. Precisely the same definition will serve equally well for 
Light, if “ light ” be substituted for “ heat, and “ luminous ” for 
“ hot.” It would then read thus : — Light is a very brisk agitation 
of the insensible parts of the object, which produces in us that 
sensation from whence we denominate the object luminous; so 
that what in our sensation is light, in the object is nothing but 
motion. 
25. A very similar definition may be assigned to Sound, which 
has, nevertheless, been declared by the authors of both essays 
to be incapable of definition, thus : — Sound is the impression on 
the proper organs of hearing produced bij certain vibratory move- 
ments of matter ; “so that what in our sensation is sound, in the 
object is nothing but motion.” 
26. Sonorous vibrations may enter the ear of the deaf man, 
and it may be that the tympanum may respond to them, while the 
organic lesion happens to be more deeply seated, but he will 
tell you there is no sound ; similarly, vibrations of another kind 
may enter the eye, and paint their perfect picture on the retina, 
but if the optic nerve have lost its function, the blind one will 
tell you there is to him no light. 
27. The correctness of Locke’s definition of heat has been 
remarkably confirmed by a bold and hazardous experiment 
performed on himself bv Professor Tyndall, which he most 
judiciously recommends not to be repeated. If a concave 
reflector be suitably placed behind the luminous carbon-points 
of an electric lamp, the rays of light and heat will be concentrated 
in a powerful focus at a distance of a few inches in front of the 
lamp. If the eye were so placed that this focus of rays would 
fall on the retina, there can be little doubt that actual dis- 
organisation of that structure would ensue. By placing a vessel 
formed of parallel plates of glass containing a sufficiently strong 
solution of iodine in carbonic bisulphide between the lamp and 
the focus, the whole of the luminous rays may be intercepted, 
