TESTS OF ACCURACY OF HEAT-MEASURING APPARATUS. 175 



oxygen, the combustion of alcohol is also used to check the accuracy 

 of the respiration apparatus. Such experiments, as well as the kind of 

 alcohol used and determination of its specific gravity, have already been 

 considered in detail (see pp. 96-105). 



For the purpose of checking the apparatus as a calorimeter, a knowl- 

 edge of the heat of combustion of the alcohol used is essential. 



HEAT OF COMBUSTION OF ALCOHOL,. 



For the determination of the heat of combustion we resort to direct 

 combustions in the bomb calorimeter. A large number of such com- 

 bustions have been made in this laboratory. Since absolute alcohol 

 absorbs water rapidly from the air, we have prepared aqueous solutions 

 of varying degrees of strength for use in these tests. 



A known weight of alcohol is placed in small gelatin capsules, such 

 as are used frequently for the administration of medicine. When gela- 

 tin capsules are used there is no loss by volatilization, and as the heat 

 of combustion of the gelatin is quite constant (about 4.452 calories per 

 gram) the absolute amount of heat introduced with the alcohol can be 

 determined with considerable accuracy. The capsules weigh not far 

 from 0.3 gram, thus introducing about 1.3 calories. 



Inasmuch as in the combustion of alcohol a certain portion of the 

 oxygen combines with the hydrogen of the alcohol to form water, which 

 is condensed inside the bomb, the gas in the bomb is at a somewhat 

 less pressure at the end than at the beginning of the combustion. The 

 slight expansion of the residual gas, as a result of a diminished press- 

 ure, produces a cooling effect, and the heat of combustion of the alcohol 

 must be corrected for constant pressure. It is necessary, therefore, to 

 add to the heat of combustion of the alcohol a certain factor which is 

 obtained in the following manner : To reduce the molecular heat of 

 combustion of a solid or liquid, the formula of which is C n H p N r O q , from 

 that at constant volume to that at constant pressure, a correction would 

 be added of (^ p q r)T calories, where T equals the absolute tem- 

 perature of the calorimeter. 1 To reduce the specific heat of combustion 

 at constant volume to that at constant pressure, the amount to be added 

 is, therefore, (% p q r) T -=- M, where M equals the molecular weight 

 of the substance. For alcohol this correction amounts to 13 calories per 

 gram. The corrected heat of combustion of anhydrous ethyl alcohol is 

 taken in this discussion as 7.080 calories per gram. 



1 For discussion of this point see Atwater and Snell, Jour. Am. Chem. Soc., 25, 

 I93, PP- 690, 691. 



