30 CORRELATION OF PHYSICAL FORCES. 



scaffolding which is seldom requisite, and should be sparingly 

 used, even in the early stages of discovery. As an instance 

 I think a striking one, of the injurious effects of this, I will 

 mention the analogous doctrine of ' invisible light ;' and I do 

 this meaning no disrespect to its distinguished author, any 

 more than, in discussing the doctrine of latent heat, I can be 

 supposed, in the slightest degree, to aim at detracting from 

 the merits of the illustrious investigators of the facts which 

 that doctrine seeks to explain. Is not * invisible light ' a con- 

 tradiction in terms ? Has not light ever been regarded as that 

 agent which affects our visual organs ? Invisible light, then, 

 is darkness, and if it exist, then is darkness light. I know it 

 may be said, that one eye can detect light where another 

 cannot ; that a cat may see where a man cannot ; that an 

 insect may see where a cat cannot ; but then it is not invisible 

 light to those who see it : the light, or rather the object seen 

 by the cat, may be invisible to the man, but it is visible to 

 the cat, and, therefore, cannot abstractedly be said to be 

 invisible. If we go further, and find an agent which affects 

 certain substances similarly to light, but does not, as far as we 

 are aware, affect the visual organs of any animal, then is it 

 not an erroneous nomenclature which calls such an agent 

 light ? There are many cases in which a deviation from the 

 once accepted meaning of words has so gradually entered into 

 common usage as to be unavoidable, but I cannot but think 

 that additions to such cases should as far as possible be 

 avoided, as injurious to that precision of language which is 

 one of the safest guards to knowledge, and from the absence 

 of which physical science has materially suffered. 



Let us now shortly examine the question of latent heat, 

 and see whether the phenomena may not be as well, if not 

 more satisfactorily, explained without the hypothesis of latent 

 matter, an idea presenting many similar difficulties to that of 

 invisible light, though more sanctioned by usage. Latent 

 heat is supposed to be the matter of heat, associated, in a 

 masked or dormant state, with ordinary matter, not capable 

 of being detected by any test so long as the matter with 

 which it is associated remains in the same physical state, but 

 communicated to or absorbed from other bodies, when the 



