PRACTICAL EXERCISES 721 



ii.* Measurement of the Quantity of Heat given off in Respiration. 



This may be done approximately as follows: Put in the inner copper 

 vessel, A, of the calorimeter shown in Fig. 224 (p. 682) a measured 

 quantity of water sufficient to completely cover the series of brass discs. 

 Place A in the wide outer cylinder, the bottom of which it is prevented 

 from touching by pieces of cork. The outer cylinder hinders loss of 

 heat to the air. Suspend a thermometer in the water through one of the 

 holes in the lid. In the other hole place a glass rod to serve as a stirrer, 

 Read off the temperature of the water. Put the glass tube connected 

 with the apparatus in the mouth, and breathe out through it as regu- 

 larly and normally as possible, closing the opening of the tube with 

 the tongue after each expiration and breathing in through the nose. 

 Continue this for five or ten minutes, taking care to stir the water fre- 

 quently. Then read off the temperature again. If W be the quantity 

 of water in c.c., and t the observed rise of temperature in degrees Centi- 

 grade, Wt equals the quantity of heat, expressed in small calories 

 (p. 675), given off by the respiratory tract in the time of the experiment, 

 on the assumptions (i) that all the heat has been absorbed by the water, 

 (2) that none of it has been lost by radiation and conduction from the 

 calorimeter to the surrounding air. Calculate the loss in twenty -four 

 hours on this basis ; then repeat the experiment, breathing as rapidly 

 and deeply as possible, so as to increase the amount of ventilation. 

 The quantity of heat given off will be found to be increased, f 



In an experiment of short duration (2) is approximately fulfilled. 

 As to (i), it must be noted that in the first place the metal of the 

 calorimeter is heated as well as the water, and the water-equivalent 

 of the apparatus must be added to the weight of the water (p. 676). 

 The water-equivalent is determined by putting a definite weight of 

 water at air temperature T into the calorimeter, and then allowing a 

 quantity of hot water at known temperature T' to run into it, stirring 

 well, and noting the temperature of the water when it has ceased to 

 rise. Call this temperature T". Enough hot water should be added 

 to raise the temperature of the calorimeter about 2 C. The quantity 

 run in is obtained by weighing the calorimeter before and after the 

 hot water has been added. Suppose it is m. Let the mass of the cold 

 water in the calorimeter at first be M, and let M'=the mass of water 

 which would be raised i C. in temperature by a quantity of heat suffi- 

 cient to increase the temperature of all the metal, etc., of the calorimeter 

 by i in other words, the water-equivalent of the calorimeter. 



The rnass m of hot water has lost heat to the amount of m (T' T"), 

 and this has gone to raise the temperature of a rnass of water M, 

 and metal equivalent to a mass of water M', by (T" T) degrees. 

 .-. m (T' T") = M(T" T) + M'(T" T). Everything in this equation 

 except M' is known, and .'. M', the water-equivalent of the calorimeter, 

 can be deduced, and must be added in all exact experiments to the mass 

 of water contained in it. 



Secondly, all the excess of heat in the expired over that in the inspired 

 air is not given off to the calorimeter, for the air passes out of it at a 

 slightly higher temperature than that of the atmosphere. At the 

 beginning of the experiment this excess of temperature is zero. If 

 at the end it is i C., the mean excess 13 0-5 C. Now, when respiration 



* This experiment is given as an example of a simple calorimetric measure- 

 ment, which can be easily performed with sufficient accuracy by students, 

 and involves the essential principles of such determinations. 



f The average heat-loss by the lungs for 51 men (calculated for the 24 hours) 

 was 312,000 small calories for normal, 919,000 for the fastest, and 195,000 for 

 the slowest breathing. 



