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A MANUAL OF PHYSIOLOGY 



case liquefaction of ice, in the other evaporation of ether is taken 

 as token and measure of heat received by the measuring substance, 

 the number of units of heat corresponding to liquefaction of unit 

 mass of ice or evaporation of unit mass of ether being known. The 

 unit generally adopted in the measurement of heat is the quantity 

 required to raise the temperature of a kilogramme of water i C., 

 which is called a calorie, or kilocalorie, or large calorie. The 

 thousandth part of this, the quantity needed to raise the temperature 

 of a gramme of water by i , is termed a small calorie or millicalorie. 



In the calorimeters which have been chiefly used in physiology 

 either water or air has been taken as the measuring substance. The 

 simplest form of water calorimeter is a box with double walls, the 



space between which is filled with 

 a weighed quantity of water. The 

 animal is placed inside the vessel, and 

 the temperature of the water noted at 

 the beginning and end of the experi- 

 ment. Suppose that the quantity of 

 water is 10 kilos, and that the tem- 

 perature rises i in thirty minutes, 

 then the amount of heat lost by the 

 animal is 10 calories in the half-hour, 

 or 480 calories in the twenty-four 

 hours ; and if the rectal temperature is 

 unchanged this will also be the amount 

 of heat produced. 



Here we assume (i) that all the heat 

 lost by the animal has gone to heat the 

 water and none to heat the metal of 

 the calorimeter 5(2) that none has been 

 radiated away from the outer surface 

 of the latter. The first assumption 

 will seldom introduce any sensible error 

 in a prolonged physiological experi- 

 ment ; but it is very easy to determine 

 by a separate observation the water- 

 equivalent of the calorimeter that is, 

 the quantity of water whose tempera- 

 ture will be raised i by a quantity of 

 heat which just suffices to raise the 

 temperature of the metal by i 

 (p. 613). Then the water-equivalent 

 is added to the quantity of water 

 actually present, and the sum is multiplied by the rise of tempera- 

 ture. If the temperature of the room is constant, as will be 

 approximately the case in a cellar, any error due to interchange of 

 heat between the calorimeter and its surroundings may be eliminated 

 by making the initial temperature of the water as much less than that 

 of the air as the final temperature exceeds it. Then if the loss of 

 heat by the animal is uniform, as much heat is gained during the first 

 half of the experiment by the calorimeter from the air as is lost by 

 it to the air during the last half. Or, without lowering the tem- 

 perature of the water, the amount of heat lost by the calorimeter 

 during an experiment may be previously determined by a special 

 observation, and added to the quantity calculated from the observed 

 rise of temperature. Or, finally, two similar calorimeters may be 



FIG. 194. RESISTANCE THER- 

 MOMETER FOR MEASURING 

 TEMPERATURE OF SKIN. 



G, grating of lead-paper, at- 

 tached to a cover-slip, and 

 mounted on a holder ; W, W, 

 wires to the Wheatstone's bridge. 

 An increase of temperature 

 causes an increase in the re- 

 sistance of the lead. The balance 

 of the bridge is thus disturbed. 

 By experimental graduation the 

 temperature value of the deflec- 

 tion, or of the change of resist- 

 ance that balances it, is known 

 (p. 617). 



