68 ANIMAL CHEMISTRY LECTURE IV. 



with, and convertibility into, one another of which no better 

 illustration perhaps can be chosen than a steam-engine ; and the 

 philosophical toy upon the table, known as Hero's celipile, will 

 answer our purpose perfectly well. On lighting a fire under the 

 boiler of a steam-engine, some of the heat liberated in the 

 furnace gradually passes into the water ; and as each successive 

 increment of heat is absorbed from the furnace, the temperature 

 of the water rises until it arrives at a certain point. During the 

 time the water takes in rising from its original temperature up to 

 the boiling point, heat is being conveyed into it, at what may be 

 regarded as an uniform rate. When, however, a certain degree 

 of heat is reached, the temperature of the water no longer rises, 

 although the heat of the furnace continues passing into it at the 

 same rate as before ; but now some of the heat absorbed, instead 

 of being manifested in the form of heat, appears in the form of 

 motion, and the steam-engine begins to work. Just as in the 

 oelipile before you, the heat of the spirit -flame was passing into 

 the water for several minutes with no other effect than that 

 of raising its temperature; whereas^ after the lapse of several 

 minutes, the heat of the flame still passing into the water ceased 

 to increase its temperature in the slightest degree, but caused 

 instead a rapid rotatory motion of the instrument. Thus, we 

 perceive generally that the development of motion is consequent 

 upon an absorption of heat, and conversely, we shall find that an 

 arrest of motion is tantamount to the liberation of heat. For 

 example, if we employ our steam power in drilling a piece of 

 metal, the motion of the engine is lessened by the friction of the 

 drill, and a certain amount of heat is thereby generated. The 

 original heat of the furnace, absorbed by the boiling water with- 

 out any increase of its temperature, produces a less amount of 

 motion than formerly, but the diminution of motion is supple- 

 mented by a remote increase of heat less of the heat which 

 passes into the water being now manifested in the form of mo- 

 tion, but the difference reappearing at a distance in its primi- 

 tive form of heat. 



(7 1 .) Similarly when a bullet strikes upon an iron target, its 



