Ten milligrams of iron, on complete combustion, will give sixteen calories.

In the combustion of substances containing nitrogen, or in case the free nitrogen of the air be not wholly expelled from the apparatus before the burning, nitric acid is formed which is dissolved by the water produced.

The heat produced by the solution of nitric acid in water is 14.3 calories per gram molecule. The quantity of nitric acid formed is determined by titration and a corresponding reduction made in the total calculated calories.

In the titration of nitric acid it is advisable to make use of an alkaline solution, of which one liter is equivalent to 4.406 grams of nitric acid. One cubic centimeter of the reagent is equivalent to a quantity of nitric acid represented by one calorie.

Since the materials of which the bomb is composed have a specific heat different from that of water, it is necessary to compute the water thermal value of each apparatus.

The hydrothermal equivalent of the whole apparatus is most simply determined by immersing it at a given temperature in water of a different temperature.[581] With small apparatus this method is quite sufficient, but there are many difficulties attending its application to large systems weighing several kilograms. In these cases the hydrothermal equivalent may be calculated from the specific heats of the various components of the apparatus.

In calculating these values the specific heats of the various components of the apparatus are as follows:

Brass0.093
Steel0.1097
Platinum0.0324
Copper0.09245
Lead0.0315
Oxygen0.2389
Glass0.190
Mercury0.0332
Hard rubber 0.33125

Example.—It is required to calculate the hydrothermal value of a calorimeter composed of the following substances:

Hydrothermal
value.
Steel bomb and cover, 2850 grams × 0.1097312.65grams.
Platinum lining, capsule and wires, 120 grams × 0.03243.89”
Lead washer, 100 grams × 0.03153.15”
Brass outer cylinder, 500 grams × 0.09346.50”
Mercury in thermometer, 10 grams × 0.03320.33”
Glass (part of thermometer in water), 10 grams × 0.191.90”
Brass stirring apparatus (part in water), 100 grams × 0.0939.30”
Total water value of system 377.72”