1O2 ANIMAL CHEMISTRY LECTURE Y. 



place instantaneously, with evolution of sufficient heat to render 

 the leaf luminous with vivid combustion, in fact as you per- 

 ceive. Now, the amount of heat given out under these opposite 

 circumstances is identical ; the only difference is in its intensity 

 in the quantity of heat associated with a given quantity of 

 matter at a given moment. In the last case, the action being 

 instantaneous, and the quantity of matter to be heated very 

 small, we have what we call an intense heat that is, the mo- 

 mentary association of a large quantity of heat with a small 

 quantity of matter ; whereas in the other case, the action being 

 gradual, the development of heat is likewise gradual, spread over 

 a long period of time, and associated with a large quantity of 

 matter, the increased temperature of which is, therefore, at no 

 one moment very perceptible. The terms quantity and intensity 

 of heat are strictly analogous to the terms quantity and velocity 

 of motion. In a pound weight of iron raised to 1000 the 

 melting point of silver, or ten pounds weight raised to 100 

 the boiling point of water, the quantity of heat capable of being 

 imparted say, to a gallon of ice-cold water is substantially the 

 same, though the intensity of the heat is ten times as great in the 

 one case as the other ; just as the quantity of motion is the same 

 in a pound weight moving at the rate of I ooo feet, or a ten-pound 

 weight moving at the rate of 100 feet per second, although the 

 velocity of motion is ten times as great in the one case as the 

 other. We come, then, to this conclusion that chemical action, 

 whether rapid or slow, provided only that the same substances 

 react and the same products result, always furnishes the same 

 amount of heat. 



(109.) Now, let us apply this fundamental law of combination 

 to the fuel-constituents of our tissues. If we inflame a mixture 

 of hydrogen and oxygen gases, the combination and consequent 

 evolution of heat being instantaneous, we obtain, indeed, the 

 highest degree of temperature capable of being produced by 

 direct chemical action. On the other hand, if we take the same 

 mixture of oxygen and hydrogen gases, and cause them to unite 

 slowly by means of spongy platinum, the oxidation and develop- 



