TESTS OF ACCURACY OF HEAT-MEASURING APPARATUS. 173 



Graetz quotes the results obtained by Joule, Rowland, and Miculescu. 

 The more recent investigations of Griffiths, Schuster and Gannon, and 

 Callendar and Barnes are not given. Joule's value for J is a little smaller 

 than Rowland's and the recent values found by the electrical method. 

 Graetz takes the mean of the three values quoted, and the lower value 

 of Joule's result makes the mean a little lower, namely, 4.177 x io 7 at 

 15. The reciprocal of this is 0.2394, the value used by Jaeger and von 

 Steinwehr and Fischer and Wrede. 



According to Dr. Rosa, the best principle would be to use the number 

 0.2385 at 15 and then correct the number of gram-degrees measured to 

 calories at 15 by multiplying by the ratio of the specific heat at the 

 given temperature to that at 15. 



The importance of the electrical unit and conversion factor in connec- 

 tion with the experiments with the respiration calorimeter is seen when 

 it is considered that, given accurate electrical units aud factors, it is 

 possible to verify the bomb calorimeter by the respiration calorimeter. 

 By burning alcohol in the bomb calorimeter a certain heat of combustion 

 is obtained, and if the alcohol is then burned in the respiration chamber, 

 which has been calibrated and standardized by the electrical method, 

 obviously the same heat of combustion determined by both forms of 

 calorimeter is a verification of the bomb. 1 



It is furthermore significant that the difference between the heat of 

 combustion of cane sugar, naphthalene, benzoic acid, and other standard 

 materials, when determined by the bomb calorimeter used in Middle- 

 town and when determined by Fischer and Wrede, is exactly propor- 

 tional to the difference between the two conversion factors used. Pend- 

 ing a revision of the electrical units by the National Bureau of Standards, 

 we use here the factor 0.2385 for converting watt-seconds to calories 

 at 15. 



LENGTH AND DURATION OF EXPERIMENTS. 



After the coil and connections are properly installed inside the 

 chamber the Switch is closed, and the water current passing through 

 the heat- absorbers is regulated so that the heat is brought away at the 

 same rate at which it is generated. After an hour or two, during which 

 period the apparatus comes into equilibrium, the experiment proper is 

 begun. The experiment lasts usually from eight to twelve hours, during 

 which time the current is measured by the milli-ammeter and is kept 



1 For a discussion of the verification of the bomb calorimeter by the respiration 

 calorimeter see Atwater and Snell, Jour. Am. Chem. Soc. (1903), 25, p. 698. 



