BERTHELOT'S CALORIMETER. 51 



sary condition that the temperature, and consequently the 

 precision, attain a high degree. For solids and also for coal 

 Berthelot uses bombs containing 400 to 600 cubic centimetres 

 (24 to 37 cubic inches), placed in a calorimeter of 2000 grams 

 (4.4 Ibs.) of water. 



To determine the heat of combustion of coal, for instance, 



FIG. 22. BERTHELOT BOMB. 



it must be previously reduced to powder in order to have a 

 sample whose cinder is known. As all kinds of coal do not 

 burn completely in this state, they are formed into pastilles,* 

 which are weighed and burnt. They are put on a platinum 

 grating or foil, placed on the support 55 (Fig. 21), over 



* We obtain very resisting pastilles or briquettes from fat coals by 

 simple compression in a pastille or suppository mould such as used by 

 druggists. With lean coals, or anthracite, the pastilles are'too friable and 

 burn incompletely. This is easily remedied by mixing with a small 

 quantity of silicate of soda solution. Several of them should be made at 

 a time, the cinders of some being determined to obtain a mean and the 

 others burnt in the bomb. They may contain about I gram of pure coal. 



