CHAPTER V. 

 CALORIMETERS WITH CONSTANT VOLUME. 



THE results obtained with a calorimeter of constant volume 

 are not exactly the same as those obtained with one of con- 

 stant pressure ; but for solid or liquid substances the difference 

 is too small to consider, since the volume, as well as that of 

 the water produced, is inconsiderable in relation to the volume 

 of gas employed. As regards the correction for contraction 

 and expansion of the gases, they also are inconsiderable. 



In his Traite de Mfcanique Berthelot has shown that 

 the heat generated by a reaction between gases at constant 

 pressure is equal to the heat of combination at constant 

 volume at any temperature whatever, increased by the pre- 

 ceding product counting from absolute zero ; and he gives the 

 following formula for passing from one system to the other : 



QT P = QT V + 0.5424^- N' 



QT P being the heat generated by the reaction at constant 

 pressure, and at the temperature T counting from ordinary 

 zero; QT V , the heat generated by the reaction at same tem- 

 perature and constant volume; N, the number of units of 

 molecular volume occupied by the components, these being 

 taken according to usage equal to 22.32 litres under normal 

 pressure at o ; N', the corresponding number of units of 

 molecular volume occupied by the product of the reaction. 



As example, take the combustion of carbonic oxide at 15. 

 Then we have 



CO + O = CO 8 generates at constant volume 68 calories.* 



* These numbers refer to molecular weights. 



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