206 WELLS'S NATURAL PHILOSOPHY. 



_ 453. Caloric is the general name given to 



the physical agent whicli produces the sensa- 

 tion of warmth, and the various effects of heat observed 

 in matter. 

 HoTT- Is heat 454. The quantity of heat observed in dif- 



measured? fereut substanccs is measured, and its effects 

 on matter estimated, only by the change in bulk, or ap- 

 pearance, which different bodies assume, according as heat 

 is added or subtracted. 

 What Is tem- 455. The degree of heat by which a body 



perature? jg affcctcd, or the sensible heat a body con- 

 tains, is called its Temperature. 



456. Cold is a relative term expressing only 



%Vhat is cold ? . -^ . ° , 



the absence of heat m a degree ; not its total 

 absence, for heat exists always in all bodies. 

 What distin- 457. Heat possesses a distinguishing char- 

 ncteristfc'^docs ^ctcristic of passing through and existing in 

 heat possess? ^^^ kiuds of matter at all times. So far as we 

 know, heat is everywhere present, and every body that 

 exists contains it without known limits. 



Ice contains heat in large quantities. Sir Humphrey Davy, by friction, ex- 

 tracted heat from two pieces of ice, and quickly melted them, in a room cooled 

 below the freezing-point, by rubbing them against each other. 



In what man- 458. Tlio tcudency of heat is to diffuse, or 

 diffufe,"^ oT' spread itself among all neighboring substances, 

 spread itself? until all havc acquired the same, or a uniform 

 temperature. 



A piece of iron thrust into burning coals becomes hot among them, because 

 the heat passes from the coals into the iron, until the metal has acquired aa 

 equal temperature. 



When do we 459. Wlicn thc hand touches a body having 

 caiia body hot r ^ higher temperature than itself, we call ifc 

 hot, because on account of the law that heat diffuses itself 

 among neighboring bodies until all have acquired the 

 same temperature, heat passes from the body of higher 

 temperature to the hand, and causes a peculiar sensation, 

 which we call warmth. 



400. When we touch a body having a temperature 



