203.]

HEAT EVOLVED DURING CHEMICAL ACTION.

The

FIG. 158.

427

mann (Ann. Chim. Phys. 1852 [3], xxxiv. 357; xxxvi. 5, and 1853; xxxvii. 406). These experiments were conducted in many cases upon a larger scale than those of Andrews, and with a much more elaborate apparatus. It is satisfactory to find, however, that their experimental results generally agree pretty closely with those of Andrews, although they differ from him in some of their deductions. The essential part of Favre and Silbermann's apparatus was a vessel of brass gilt, a, fig. 158, in which the combustions were performed; this vessel was immersed in a calorimeter, b, of silvered copper, which contained about 2 litres of water. calorimeter was supported in an outer vessel, c, lined with swan's down, and this case was itself surrounded by an outer double envelope, d, filled with water. It was found that by these means the loss of heat from the influence of the external atmosphere was reduced to a very small and measurable amount. When the combustions were performed in oxygen, this gas, previously dried, was allowed to flow into the combustionchamber by the tube e, and the gases produced, together with the superfluous oxygen, were forced, before their exit from the apparatus, to traverse a spiral tube of thin copper, f, so that they might be completely cooled down to the temperature of the water in the calorimeter, b; g g is an agitator for ensuring uniformity of temperature in the water of the calorimeter.

Solid bodies were kindled by the introduction of small pieces of burning charcoal; liquids were burnt in small lamps with asbestos wicks, and gases were introduced by a jet previously set on fire. The apparatus in the figure shows the arrangement for burning carbon; the scale of the thermometers employed allowed a variation of 10 of 1° C. to be estimated. In most cases the weight of the substance burned was ascertained by collecting and weighing the products of combustion.

(203) Quantities of Heat evolved during Combustion.-The preceding Table is compiled chiefly from the results of Dulong, Andrews, and Favre and Silbermann. It is founded upon the direct results obtained by the rapid combustion, in oxygen, of the various substances enumerated in the first column. The heat unit adopted is the one proposed by Dulong, viz., the quantity of heat

428

HEAT EVOLVED DURING CHEMICAL ACTION.

[203.

required to raise 1 gramme of water 1° C., or rather from o° C. to 1° C. The second column indicates the units of heat evolved during the act of combustion; or the number of grammes of water which would be raised from o° C. to 1° by the combustion of 1 gramme of each substance. The third column indicates the number of grammes of water heated to the same amount by the combination of 1 gramme of oxygen with each body; and the fourth column (the calorific equivalent) is obtained by multiplying the numbers in the third column by 8 (the equivalent number of oxygen).

Quantities of Heat disengaged by the Action of Chlorine.

Quantities of Heat disengaged by the Action of Bromine and Iodine.

* Experimental numbers.

+ Transactions of the Royal Irish Academy, 1843, xix. 393.

In some cases the quantity of the combustible was weighed or measured, and in others the quantity of oxygen absorbed was

204.]

HEAT EVOLVED DURING CHEMICAL ACTION.

:

429

determined in the first class of experiments the direct result is contained in the second column, the numbers in the other columns being calculated; and in the second class the experimental numbers are found in the third column. The numbers marked with an asterisk are experimental ones.

The preceding Tables contain the results of a similar series of experiments, in which chlorine, bromine, and iodine were employed instead of oxygen.

From an inspection of these Tables it may be gathered that the quantity of heat disengaged by the following bodies, in their ordinary physical state, during their combination with an equal quantity of oxygen, is nearly the same: viz., hydrogen, carbonic oxide, cyanogen, iron, and tin, to which also may be added stannous oxide and phosphorus, though the heat disengaged by the body last named is somewhat higher than that furnished by any one of the others. If, however, to these numbers the corrections due to the change in the physical state of the products could be applied, the same coincidence would not be observed; but although the trustworthy numerical data required for making these corrections do not exist, it is quite obvious that when equivalent quantities of the different elements unite with equal quantities of oxygen without undergoing change in their physical state, they emit specific, but different amounts of heat. Sulphur, copper, and cuprous oxide disengage little more than half the heat of the substances just mentioned, and carbon is intermediate between these two groups. Zinc gives out more heat than either, and

potassium more than zinc.

(204) Influence of Dimorphism.-According to the experiments of Favre and Silbermann, equal quantities of the same substance, when in different allotropic conditions, evolve somewhat different amounts of heat during combustion; the modification which is least dense and has the highest specific heat evolving the largest quantity of heat when burned. The following results with carbon, sulphur, and phosphorus, in different states, may be given in illustration of this point :

:

430

HEAT EVOLVED DURING DECOMPOSITION.

[204.

Similar differences were observed when different forms of the same compound body were submitted to experiment. According to these observers, heat was evolved during the conversion of aragonite into calc-spar: this is somewhat remarkable, for the density of calc-spar is less than that of aragonite, and hence, from analogy, an absorption of heat was rather to be looked for in this change.

(205) Heat evolved in certain Cases during Decomposition.— In the experiments of Dulong it appeared that when carbonic oxide, or hydrogen, was burned in nitrous oxide, a larger amount of heat was evolved than when the same quantities of these gases were burned in oxygen: following up this observation, Favre and Silbermann were led to the remarkable conclusion, that nitrous oxide, in the act of decomposition, evolves a considerable amount of heat; and they estimate that not less than 1154 units of heat are evolved in the separation into its elements of a quantity of nitrous oxide which contains 1 gramme of oxygen. In the de

composition of hydric peroxide also, heat is evolved instead of being absorbed, and they estimate the heat evolved during the liberation of 1 gramme of oxygen from hydric peroxide at 1363 beat units.

Chemists are also familiar with other cases in which decomposition is attended with disengagement of heat; as when the oxides of chlorine, and the so-called iodide and chloride of nitrogen are decomposed. In these cases evolution of light and heat occurs, although the products of decomposition occupy a larger volume than the compound which furnishes them. A still more striking evolution of heat attends the explosive decomposition of gun-cotton, although the gases produced occupy many hundred times the volume of the original substance. The latter case is particularly instructive, for it is obvious that the oxygen and carbon, although present in the compound, are each there in a form in which they retain a large share of heat, ready to be evolved when more intimate chemical union occurs; and it is by no means improbable that these apparent anomalies may be due to the apparent decompositions being truly double decompositions, two new bodies being in each case formed. For example, in the instance of hydric peroxide, the decomposition may be thus represented, H2O,O+H20,0=2H2O+O2; where the heat evolved by the union of the two atoms of oxygen may be greater than that absorbed in the decomposition of the hydric peroxide.

(206) Combustion of Compounds.-Generally speaking, the

206.]

COMBUSTION OF COMPOUNDS.

431

heat given out during the combustion of a compound body is less than that emitted by the combustion separately of a quantity of each of its constituents equal in amount to that present in the compound burnt; but this is not uniformly so, as, for instance, in the case of oil of turpentin, and of carbonic disulphide. Favre and Silbermann have examined the amount of heat developed during the combustion of many hydrocarbons and compound ethers. From these experiments it appears that polymeric bodies* do not emit equal amounts of heat during combustion; but that the denser the vapour which they furnish, the smaller is the amount of heat which they evolve in combining with equal quantities of oxygen. The following Table, which indicates the amount of heat given out by hydrocarbons polymeric with olefiant gas, distinctly shows this:

In homologoust compounds, such as the alcohols and the fatty acids, it was also found that for equal quantities of oxygen consumed, the heat of combustion was diminished the oftener that the group of elements (CH) entered into the formation of the compound.

Even in metameric bodies-which contain the same number of atoms of the same elements in their molecules, but the atoms arranged in a different order in each compound, and which yield vapours of the same density-the quantity of heat evolved during combustion is not necessarily the same: from which it would appear that differences in the molecular arrangement of the component elements, although the number of the atoms may remain unaltered, may yet produce differences in the amount of heat evolved during oxidation. For example, the following metamerides (all containing CHO2) evolve different quantities of heat:

* Bodies which contain centesimally the same proportion of the same elements but which each contain a different number of atoms in their molecule.

Bodies which have a similar constitution, but which differ in composition by a multiple of CH2.