174 A RESPIRATION CALORIMETER. 



constant by means of the variable resistance. Readings on both elec- 

 trical instruments are taken frequently to insure complete accuracy. 

 At the end of the period the time in seconds is noted and the average 

 reading of the instrument taken. The formula for computing the 

 amount of energy developed during the experiment is therefore 

 C X E X / X o. 2385 calories, in which C is the strength of the cur- 

 rent in amperes, E the fall of potential in volts, and / the time in 

 seconds. 



RESULTS OF EI.ECTRICAI, CHECK EXPERIMENTS. 



The last electrical check experiment made with the apparatus was on 

 November 22, 1904. The actual period of measurement extended from 

 i. 06 p. m. to 10.04 p. m., or 8 hours and 58 minutes. During this 

 period there was a current of 0.950 ampere passed through the coil and 

 a fall of potential of 99 volts. By using the formula given above, the 

 heat generated during this period was computed to be 723.7 calories. 

 The heat measured during this period by the respiration calorimeter 

 was 721.73 calories, or 99.72 per cent of that generated. A test con- 

 ducted a month before gave the ratio of heat measured to that generated 

 corresponding to 99.59 per cent. It thus appears that the apparatus 

 measures heat developed within it electrically with great accuracy. 



THE COMBUSTION OP ETHYL ALCOHOL AS A CHECK ON THE HEAT 



MEASUREMENTS. 



Although the electrical check experiments are carried out with great 

 accuracy, they still do not permit of the testing of the apparatus under 

 conditions approximating those in which it is used in actual experi- 

 menting, and obviously the question of the heat of vaporization of water 

 plays no r61e in the electrical check experiment. As early as 1779, 

 Crawford l endeavored to study the accuracy of the heat measurements 

 of his calorimeter by burning known weights of charcoal, lamp oil, wax, 

 and tallow inside the chamber. Subsequent experimenters have used 

 hydrogen, stearin candles, ether, and other substances. As a result of a 

 large number of experiments in which a number of different combustibles 

 were tried, we have relied upon the combustion of ethyl alcohol of known 

 water content for this purpose. Inasmuch, however, as the combustion 

 of ethyl alcohol inside the chamber results not only in an evolution of 

 heat, but also of carbon dioxide and water, and in the absorption of 



1 Experiments and Observations on Animal Heat ; see also Zeits. f. Biol. (1894), 

 30, p. 76. 



