THERMOMETRY AND CALORIMETRY 



653 



W 



Fig. 210. Resistance 



w 



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 or gramme-calorie. 



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 begin- 

 ning and end of the experiment. Suppose that the quantity of water 

 is 10 kilos, and that the temperature rises i in thirty minutes, then the 

 amount of heat lost by the animal is n 



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; (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 experiment ; but 

 it is very easy to determine by a separate 

 observation the water-equivalent of the 

 calorimeter that is, the quantity of 

 water whose temperature will be raised i 

 by a quantity of heat which just suffices 

 to raise the temperature of the metal by 

 i (p. 694). Then the water-equivalent 

 is added to the quantity of water actu- 

 ally present, and the sum is multiplied by 

 the rise of temperature. If the tempera- 

 ture 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 temperature 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 cal- 

 culated from the observed rise of temperature. Or, finally, two similar 

 calorimeters may be used, one containing the animal and the other a 

 hydrogen flame, or a coil of wire traversed by a voltaic current, which 

 is regulated so as to keep the temperature the same in the two calorim- 

 eters. From the quantity of hydrogen burnt, or electricity passed, 

 the heat-production of the animal can be calculated. 



In At water's great respiration calorimeter (Fig. 211) both the heat 

 production and the respiratory exchange are measured. The heat pro- 

 duced by the person in the calorimeter is carried away from it by a 



* The student should carefully note that when the term ' calorie ' is used 

 without qualification, a large calorie, i.e., 1,000 gramme-calories, is meant. 



Thermom- 

 eter for measuring Temperature 

 of Skin. G, grating of lead- 

 paper, attached to a cover-slip, 

 and mounted on a holder; W, 

 W, wires to the Wheatstone's 

 bridge. An increase of tem- 

 perature causes an increase in 

 the resistance of the lead. The 

 balance of the bridge is thus dis- 

 turbed. By experimental grad- 

 uation the temperature value 

 of the deflection, or of the change 

 of resistance that balances it, is 

 known (p. 699). 



