BERTHELOT'S CALORIMETER. 53 



same metal, so as to burn any oil which may have beeri taken 

 from the pump. 



Operation. At the laboratory of the College of France 

 the successive operations are as follows : 



1. Light the fire to heat the oxygen red-hot; 



2. While the gas-holder is filling with oxygen, the fuel is 

 dried ; 



3. Weigh the fuel; 



4. Place the fuel in the bomb ; 



5. Grease the cover slightly; tighten with the screw; 



6. Begin to compress the oxygen by forcing the air out 

 with a few strokes of the piston ; pump slowly to prevent 

 heating the pump ; 



7. Close the stop-cock of the pump ; break the connection 

 with the bomb> extinguish the fire, and replace the bomb on 

 its support so as to carry it to the calorimeter room ; 



8. Pour the water into the calorimetric bath. 



The apparatus is allowed to come to equilibrium, and the 

 readings of the thermometer taken for Jive: minutes. The 

 iron coil is then heated by the electric current from a small 

 bichromate battery. It takes fire and kindles the combustible, 

 which generally burns without smoke or producing any car- 

 bonic oxide, as Befthelot has shown.*/ 



The water condensed from the combustion contains small 

 quantities of nitric acid, showing imperfectly purified gas. This 

 may be determined by titratiqn, if accurate results are sought, 

 and calculated 0.227 calories per gram of HNO,. , The cor- 

 rection will be very small. A correction for the iron used 

 may be made at the rate of 1.65 calories per gram, this being 

 the heat of formation of the magnetic oxide. 



* With very fat coals it sometimes happens after a combustion that the 

 platinum shows a black or brown mark, indicating a slight deposit of black 

 or tar which has. escaped combustion. Occasionally, alao, a trace of tar is 

 found at the bottom of the bomb. These may be prevented by using a 

 grating or perforated plate instead of the foil. This detail must be attended 

 to with a new coal. 



