235 



effective motion to the indefinitely denser forms of cognisable 

 matter. 



44. It was objected by Dr. Young to the views here advo- 

 cated^ that if ordinary matter be susceptible of luminous 

 vibrations^ all bodies ought more or less to absorb lights and to 

 become luminous, just as all bodies become more or less heated, 

 by absorbing radiated heat. To this it may be replied that a large 

 number of bodies is now known to be phosphorescent after 

 exposure to light ; but that in many the duration of that 

 property is exceedingly brief : when enclosed in a glass tube, 

 and placed in a slit in a dark screen, surrounding an electric 

 light, they emit visible light only when rotated with great 

 rapidity, so that the particles may be presented to the eye 

 within the 10th or 20th of a second after their exposure to 

 light. If the velocity of rotation could be indefinitely increased, 

 it is not improbable that all substances would become luminous, 

 for it must be remembered that the 50th or 100th of a second 

 is as an age when compared with the duration of a wave of 

 light. 



45. Moreover, matter is equally capable of absorbing the 

 invisible rays, that are known by their chemical eff'ects to be 

 present in the spectrum. This has been shown by the experiments 

 of M. Niepce. An engraving, which has been placed for some 

 days in the dark, is half covered with an opaque screen, and 

 then exposed to sun-light. The engraving is then placed (with 

 the usual precautions of a photographic process) in juxta- 

 position bu.t not in contact with a piece of sensitive paper. An 

 inverted or ' negative ' image of that portion of the engraving 

 which has been exposed to light will be produced on the paper, 

 while the portion that was covered up will produce no effect. 

 Again if the , engraving after exposure be placed in contact for 

 several hours with a sheet of white paper not recently exposed 

 to light, and the latter be then applied to the sensitive paper, a 

 faint impression of the exposed portion of the engraving will 

 still be produced. 



46. Dr. McCann, having first identified heat and motion as 

 synonymous terms, impugns the theory of latent heat as 

 involving a " contradiction in terms,'^ and it is by no means 

 the first time that that theory has been put forward as a 

 stumbling-block to the dynamic theory of heat. " Latent 

 heat ^' is an unfortunate and misleading term, and has mystified 

 this writer as well as many others : it ought long ago to have 

 been discarded, together with the material theory of heat, from 

 whic?i it arose. 



47 . A much better term would be employed or occupied heat, 

 for the so-called latent heat is wholly employed or occupied in 



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