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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 by 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 Lockers definition of heat has been 

 remarkably confirmed by a bold and hazardous experiment 

 performed on himself by 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, 



