PRACTICAL EXERCISES 013 



ment will be made still more instr active by determining the total 

 nitrogen in each sample in addition to the urea. A curve showing 

 the variation in the total nitrogen can then be 'plotted on the same 

 paper as the urea curve, and a table calculated giving the percentage 

 of the total nitrogen contained in the urea for each day of the 

 experiment. 



9. 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 respiration calorimeter (Fig. 197, p. 579) 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 regularly 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 frequently. Then 

 read off the temperature again. If W be the quantity of water 

 in c.c., and / the observed rise of temperature in degrees Centigrade, 

 W/ equals the quantity of heat, expressed in small calories (p. 574)* 

 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.* 



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. 574). 

 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 sufficient to increase the temperature of all 

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

 equivalent of the calorimeter. 



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

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



20*75 grammes (minimum 9*517 grammes, maximum 32*857 grammes) ; 

 on a diet rich in protein, average 38*83 grammes (minimum 23*265 grammes, 

 maximum 67*82 grammes). 



* The average heat-loss by the lungs for 5 1 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. 



