26 HEAT. 



What is the 43. THEORY OF VIBRATION. Another 

 bration? view, corresponding to the second view ol 

 light, is, that heat is not a fluid, but, like light, the result 

 of vibration in the ether which is every where present. 

 The vibrations which produce the sensation of heat 

 are, of course, different from those which produce that 

 of light, as the movements of the air which produce 

 heavy and sharp sounds are different. We must sup- 

 pose, indeed, in the former case, that a much greater 

 difference exists. But it is assumed that both are the 

 result of vibrations of some kind. 



44. ILLUSTRATION. When a bell is struck 

 Give the illus- ^ s vibrations are communicated to the air, 



tration. 



and so to the ear, producing the effect of 

 sound. So, according to this view, vibrations of a pe- 

 culiar kind are caused by some means in the sun, and 

 all sources of heat, and, being rapidly transmitted 

 through the ether, produce, when they fall upon our 

 bodies, the sensation of heat. So the bar heated at 

 one end becomes hot at the other, because certain vi- 

 brations, originated in the fire, are gradually transmit- 

 ted through the ether, and the iron which it pervades, 

 to the other end. 



45. THE FACTS ARE DEFINITELY KNOWN. 



What is the 



limit of our It may seem strange to the reader that 



C ct? there should be this doubt in relation to so 

 common a subject as heat. But there is a 

 similar limit to our knowledge in most of the sciences. 

 In physiology, for example, we know that muscle, and 

 bone, and other parts of the body, are produced from 

 the blood, and that life, or vital force, is essential to 



