Page:Outlines of Physical Chemistry - 1899.djvu/182

 164 OUTLINES OF PHYSICAL CHEMISTRY

The difference found evidently corresponds to the difference which existed between the initial states, and ex- presses the heat of formation of the organic substance from its elements.

In a word, the heat of combustion of an organic substance is lower than that of the elements which it con- tains by that quantity of heat which is evolved when the elements unite to form the compound.

For a compound CJB*0*, the heat of formation is there- fore equal to

«(0,O 1 ) + |(H„O)-/(0^ f O.)-

For carbon, taken as diamond, (C, 2 ) = 94*81 Cal. For liquid water (H 2, 0) = 69 Cal. If we wish to know the heat of formation from amorphous carbon, then we must take (C, 2 ) = 97'65 Cal. (Berthelot's figures).

The third term, /(C x H y O a ) expresses the value found experimentally for the heat of combustion of the organic substance.

The heat of combustion has been determined for a large number of substances. But the numbers found had rather a doubtful value, and might, particularly for com- pounds containing sulphur, chlorine, &c., be infected with a considerable error. Since the introduction of Berthelot's calorimetric bomb, however, it has been possible to determine the heat of combustion of organic substances more accurately.

The form of this apparatus is shown in fig. 87.

The apparatus consists of a very strong vessel and cover of steel, nickelled on the outside and lined in the interior with some non-oxidisable material, such as platinum, gold, or enamel (Mahler). The cover is fitted on to the vessel by means of a piece of steel screwed to the body of the bomb. The cover has several appendages. The central tubulus a is made of steel, and contains a screw which serves as a valve and as the inlet pipe for the

�� �� �