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 light, and to 
become luminous, just as all bodies become more or less heated, 
by absorbing radiated heat. Tq 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 effects 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 but not in contact with a piece of sensitive paper. An 
inverted or f 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 JJ> 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 
which 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 
r 2 
