Human Physiology. 101 



against a target is very hot. When motion is stopped, or even 

 impeded, heat is produced in proportion to the destroyed 

 momentum. A blacksmith hammers a soft nail rod red hot to 

 light his pipe and fire. A mass of stone passing through our 

 atmosphere is heated to whiteness by the friction of the atmos- 

 phere, and becomes a shooting-star, or meteor. I believe, 

 however, that in the case of the hammered iron, the heat is 

 that which, acting as a repulsive force, kept its atoms apart. 

 Driven together, this latent heat becomes active, just as when 

 air, suddenly compressed in a tube, gives out heat enough to 

 light a bit of tinder. But heat is a form of force, if not always 

 a form of motion. It may be the expansive force, or repulsive 

 power of all bodies. It may constitute, as it undoubtedly 

 increases, the atmosphere of repulsion which surrounds every 

 atom of the universe, and keeps it wide asunder from every 

 other. Heat acts with an immense force, for it expands the 

 hardest substances in nature mountains of granite, and masses 

 of the most tenacious metals. It expands solids to liquids, 

 and liquids to gases. Heat added to water gives us the tre- 

 mendous powers of steam. Heat maintains the fluidity of the 

 earth, and shows its tremendous forces in volcanic eruptions 

 and earthquakes, The heat of the sun, with its light, covers 

 the earth with vegetable and animal life. 



What, then, is heat? Not a substance streaming from the 

 sun, and fires, and heated bodies ; not an emanation ; but the 

 vibrations of an elastic fluid like the air, but immeasurably 

 finer, and having very different qualities. The sun, and all 

 sources of heat and light, excite vibratory movements of this 

 ether, as sonorous bodies excite in the atmosphere vibrations 

 of sound. These vibrations are communicated to the atoms 

 of matter, and the movement produced gives the sensation of 

 heat. This is the latest scientific explanation ; but it is not 

 very satisfactory. How can the expansive power of heat, as 

 shown, for example, in the force of steam, come from the 



