4:2 CORRELATION OF PHYSICAL FORCES. 



generally considered as strongly in favour of that view which 

 regards heat either as actual matter, or, at all events, as a 

 substantive entity, and not a motion or affection of ordinary 

 matter. 



The hypothesis of latent matter is, I venture with diffi- 

 dence to think, a dangerous one it is something like the old 

 principle of Phlogiston, it is not tangible, visible, audible ; 

 it is, in fact, a mere subtle mental conception, and ought, I 

 submit, only to be received on the ground of absolute neces- 

 sity, the more so as these subtleties are apt to be carried on 

 to other natural phenomena, and so they add to the hypothe- 

 tical 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 au- 

 thor, any more than in discussing the doctrine of latent heat, 

 I can be supposed, in the slightest degree, to aim at detract- 

 ing from the merits of the illustrious investigators of the facts 

 which that doctrine seeks to explain. Is not ' invisible light/ 

 a contradiction 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 de- 

 viation from the once accepted meaning of words has so grad- 



