CHAPTER IV. 

 CALORIMETERS WITH CONSTANT PRESSURE. 



THE first calorimeters were of constant pressure; that is, 

 the combustion was carried on at the atmospheric pressure or 

 very near it, and did not vary from the beginning to the end 

 of the experiment. Hence the modifications in the volume 

 of the gases before and after combustion exercised no influ- 

 ence on the observed results. 



Rumford, in 1814, was the first who tried to correct 

 external influences. He employed a practical method which 

 has often been used since, and consists in giving the calo- 

 rimeter bath a temperature in the beginning of the test less 

 than that of the room, and allowing it at the close to attain 

 a temperature in the same proportion above that of the room. 

 His calorimetric apparatus was composed of a copper boiler 

 of several litres capacity, heated by an interior tube through 

 which passed the gaseous products of the combustion. The 

 combustible was burnt in a little burner placed under the 

 boiler, and the air used circulated around the heater before 

 passing to the burner, thus preventing any loss of caloric by 

 radiation. 



Dulong in 1838 used oxygen, and obtained much superior 

 results. His calorimeter consisted of a rectangular copper 

 box, 25 centimetres (about 10 inches) deep, 7.5 centimetres 

 (2.9 inches) wide, and 10 centimetres (3.9 inches) long. It 

 was closed at the upper part by a cover with a mercury seal. 



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