178 PHYSIOLOGY AND HEALTH. 



and abstracting heat renders them more solid, while adding 

 heat renders them more fluid. By a great reduction of tem- 

 perature, airs or gases can be condensed to fluids ; and, 

 during this change, heat is given out. 



417. The apparent heat of a body, as measured by the 

 thermometer, or as perceived by the touch, is not always an 

 exact measurement of the quantity of heat in that body. If, 

 for instance, we mix a pound of water heated to 100, with 

 a pound of spermaceti oil at 50, it might be supposed that 

 the temperature of the mixture would be 75, the exact me- 

 dium between them. This would be the case, if each of 

 these bodies were raised to the same temperature by the 

 same quantity of heat. But the temperature of the mixture 

 is actually 83J. If, again, the experiment be reversed, and 

 water at 50, and oil at 100, be mixed, the result is a tem- 

 perature of 66. Instead of the warmest substance losing 

 25, and the coolest gaining as much heat, we find that, in 

 the first instance, the water loses only 16|, while the oil 

 gains 33J ; and, in the other case, the oil loses 33J, and 

 the water gains 16|. That is, the quantity of heat that will 

 warm water 16 J will warm oil 33 J ; or, the water requires 

 twice as much heat as oil does, to raise it to any definite 

 temperature. It will, then, be clearly understood, that the 

 same substance has different quantities of heat in its different 

 states ; and also that one substance requires more heat than 

 another to give it the same apparent heat. 



418. The burning of wood and all other fuel shows 

 both of these principles. Oxygen exists in the air in the 

 state of gas. ( 292, p. 133.) When wood or coal is heated, 

 this oxygen combines with the carbon of the fuel, and forms 

 carbonic acid. In this process, the oxygen enters into a 

 new state, and becomes a part of a compound more dense 

 than it was before. Oh two accounts it loses heat ; 1st, 

 oxygen has greater capacity for, or holds more, heat than 

 carbonic acid gas, and therefore, when this new gas is 

 formed, the surplus heat, or that excess of heat which oxygen 

 can hold over that which the other gas can hold, must be 

 given out ; 2d, the oxygen is in a denser state when it 



