Page:Outlines of Physical Chemistry - 1899.djvu/184

 166 OUTLINES OF PHYSICAL CHEMISTRY

numbers necessary for the calculation of the true maximum temperature are thus obtained (see page 148).

To calculate the heat evolved the water equivalents of all the instruments must be known. For the bomb itself this is very considerable. It may be calculated from a very exact knowledge of the weight and specific heat of the various parts of the instrument. The water equivalent of the bomb may also be ascertained by burning in it a known weight of a substance whose heat of combustion is very exactly known (naphthalene, camphor, &c). The quantity of heat evolved is known, and the calorimetric calculation contains only a single unknown — the water-equivalent of the bomb. We may also arrive at this value simply by the method of mixture. The bomb (at a known temperature) is immersed in a known weight of water (at a somewhat higher temperature, e.g. 60°), and from the final tempera- ture of the system we may easily calculate the caloric capacity of the bomb.

The quantity of material used must be such that at least two-thirds of the oxygen remains in excess, and it is necessary to take into account the water-equivalent of the oxygen compressed in the apparatus. Further, as com- mercial oxygen (compressed in cylinders to 100 atmospheres pressure) may contain up to 10 per cent, of nitrogen, a certain quantity of nitric acid is formed by the combustion. This quantity of acid must be determined and its heat of formation deducted from the general result. The heat of combustion of the small iron spiral has also to be deducted. In order to make it unnecessary to take into account the evaporation of a part of the water produced, oxygen saturated with water vapour is used. It suffices for this to introduce into the bomb a known weight of water (which, of course, increases the water equivalent of the apparatus).

For data concerning the employment of an auxiliary com- bustible (for certain substances which do not burn easily), or the combustion of substances containing sulphur,

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