Section I.—Phenomena accompanying an Explosion.
Nature of an Explosion.
—The combination of oxygen with other substances for which it has affinity is called generally “oxidation.” The result of this combination is a new substance, and the process of change is accompanied by the liberation of heat. The quantity of heat set free when two substances combine chemically is constant, that is, it is the same under all conditions. If the change takes place within a short space of time, the heat becomes sensible; but if the change proceeds very slowly, the heat cannot be felt. The same quantity, however, is liberated in both cases. Thus, though the quantity of heat set free by a chemical combination is under all conditions the same, the degree or intensity of the heat is determined by the rapidity with which the change is effected.
When oxidation is sufficiently rapid to cause a sensible degree of heat, the process is described as “combustion.” The oxidation of a lump of coke in the furnace, for example, is effected within a short space of time, and, as the quantity of heat liberated by the oxidation of that weight of carbon is great, a high degree results. And it is well known and obvious that as combustion is quickened, or, in other words, as the time of change is shortened, the intensity of the heat is proportionally increased. So in the case of common illuminating gas, the oxidation of the hydrogen is rapidly effected, and, consequently, a high degree of heat ensues.
When oxidation takes place within a space of time so short as to be inappreciable to the senses, the process is described as “explosion.” The combustion of a charge of gunpowder, for example, proceeds with such rapidity that no interval can be perceived to intervene between the commencement and the termination of the process. Oxidation is in this case, therefore, correctly described as an explosion; but the combustion of a train of gunpowder, or of a piece of quick-match, though exceedingly rapid, yet, as it extends over an appreciable space of time, is not to be so described. By analogy, the sudden change of state which takes place when water is “flashed” into steam, is called an explosion. It may be remarked here that the application of this expression to the bursting of a steam boiler is an abuse of language; as well may we speak of an “explosion” of rock.
From a consideration of the facts stated in the foregoing paragraphs, it will be observed that oxidation by explosion gives the maximum intensity of heat.
Measure of Heat, and specific Heat.
—It is known that if a certain quantity of heat will raise the temperature of a body one degree, twice that quantity will raise its temperature two degrees, three times the quantity, three degrees, and so on. Thus we may obtain a measure of heat by which to determine, either the temperature to which a given quantity of heat is capable of raising a given body, or the quantity of heat which is contained in a given body at a given temperature. The quantity of heat requisite to produce a change of one degree in temperature is different for different bodies, but is practically constant for the same body, and this quantity is called the “specific heat” of the body. The standard which has been adopted whereby to measure the specific heat of bodies is that of water, the unit being the quantity of heat required to raise the temperature of 1 lb. of water through 1° Fahr., say from 32° to 33°. The quantity of heat required to produce this change of temperature in 1 lb. of water is called the “unit of heat,” or the “thermal unit.” Having determined the specific heat of water, that of air may in like manner be ascertained, and expressed in terms of the former. It has been proved by experiments that if air be heated at constant pressure through 1° Fahr., the quantity of heat absorbed is 0·2375 thermal units, whatever the pressure or the temperature of the air may be. Similarly it has been shown that the specific heat of air at constant volume is, in thermal units, 0·1687; that is, if the air be confined so that no expansion can take place, 0·1687 of a thermal unit will be required to increase its temperature one degree.
Heat liberated by an Explosion.
—In the oxidation of carbon, one atom of oxygen may enter into combination with one atom of that substance; the resulting body is a gas known as “carbonic oxide.” As the weight of carbon is to that of oxygen as 12 is to 16, 1 lb. of the former substance will require for its oxidation 11⁄3 lb. of the latter; and since the two enter into combination, the product, carbonic oxide, will weigh 1 + 11⁄3 = 21⁄3 lb. The combining of one atom of oxygen with one of carbon throughout this quantity, that is, 11⁄3 lb. of oxygen, with 1 lb. of carbon, generates 10,100 units of heat. Of this quantity, 5700 units are absorbed in changing the carbon from the solid into the gaseous state, and 4400 are set free. The quantity of heat liberated, namely, the 4400 units, will be expended in raising the temperature of the gas from 32° Fahr., which we will assume to be that of the carbon and the oxygen previous to combustion, to a much higher degree, the value of which may be easily determined. The 4400 units would raise 1 lb. of water from 32° to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is 0·17 when there is no increase of volume, the same quantity of heat will raise 1 lb. of that gas from 32° to 32 + 4400 0·17 = 25,914°. But in the case under consideration, we have 21⁄3 lb. of the gas, the resulting temperature of which will be 25,91421⁄3 = 9718°.
In the oxidation of carbonic oxide, one atom of oxygen combines with one atom of the gaseous carbon; the resulting body is a gas known as “carbonic acid.” Since 21⁄3 lb. of carbonic oxide contains 1 lb. of carbon, that quantity of the oxide will require 11⁄3 lb. of oxygen to convert it into the acid, that is, to completely oxidize the original pound of solid carbon. By this combination, 10,100 units of heat are generated, as already stated, and since the carbon is now in the gaseous state, the whole of that quantity will be set free. Hence the temperature of the resulting 32⁄3 lb. of carbonic acid will be
32 + 4400 + 10,100 0·17 × 3·667 = 23,516°.
It will be seen from the foregoing considerations that if 1 lb. of pure carbon be burned in 22⁄3 lb. of pure oxygen, 32⁄3 lb. of carbonic acid is produced, and 14,500 units of heat are liberated; and further, that if the gas be confined within the space occupied by the carbon and the oxygen previously to their combination, the temperature of the product may reach 23,516° Fahr.
In the oxidation of hydrogen, one atom of oxygen combines with two atoms of the former substance; the resulting body is water. As the weight of hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas will require for its oxidation 8 lb. of the latter; and since the two substances enter into combination, the product, water, will weigh 1 + 8 = 9 lb. By this union, 62,032 units of heat are generated. Of this quantity, 8694 are absorbed in converting the water into steam, and 53,338 are set free. The specific heat of steam at constant volume being 0·37, the temperature of the product of combustion, estimated as before, will be
32 + 53,338 0·37 × 9 = 16,049°.
Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb. of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat will be liberated; and further, that the temperature of the product may reach 16,049°.
Gases generated by an Explosion.
—It was shown in the preceding paragraph that in the combustion of carbon, one atom of oxygen may unite with one atom of carbon to form carbonic oxide, or two atoms of oxygen may unite with one atom of carbon to form carbonic acid. When the combination takes place according to the former proportions, the reaction is described as “imperfect combustion,” because the carbon is not fully oxidized; but when the combination is effected in the latter proportions, the combustion is said to be “perfect,” because no more oxygen can be taken up. The products of combustion are in both cases gaseous. Carbonic oxide, the product of imperfect combustion, is an extremely poisonous gas; it is this gas which is so noisome in close headings, and in all ill-ventilated places, after a blast has been fired. A cubic foot of carbonic oxide, the specific gravity of which is 0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1 lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon imperfectly oxidized will give 21⁄3 lb. of carbonic oxide, which, at the mean atmospheric pressure of 30 inches and the mean temperature of 62° Fahr., will occupy a space of 13·5 × 21⁄3 = 31·5 cubic feet. The product of perfect combustion, carbonic acid, is a far less noxious gas than the oxide, and it is much more easily expelled from confined places, because water possesses the property of absorbing large quantities of it. In an ill-ventilated but wet heading, the gas from a blast is soon taken up. Carbonic acid is a comparatively heavy gas, its specific gravity relatively to that of common air being 1·524. Hence a cubic foot at the ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the gas under the same conditions will occupy a space of 8·6 cubic feet. Thus if 1 lb. of carbon be completely oxidized, there will result 32⁄3 lb. of carbonic acid, which will fill a space of 8·6 × 32⁄3 = 31·5 cubic feet. It will be observed that, though an additional pound of oxygen has been taken up during this reaction, the product occupies the same volume as the oxide. In complete combustion, therefore, a contraction takes place.
In the oxidation of hydrogen, as already pointed out, one atom of oxygen combines with two atoms of the former substance to form water. In this case, the product is liquid. But the heat generated by the combustion converts the water into steam, so that we have to deal with this product also in the gaseous state, in all considerations relating to the effects of an explosion. A cubic foot of steam, at atmospheric pressure and a temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which, under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26 cubic feet.
Usually in an explosion a large quantity of nitrogen gas is liberated. This gas, which is not in itself noxious, has a specific gravity of 0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb. will occupy a space of 13·5 cubic feet, which are the weight and the volume of carbonic oxide. Other gases are often formed as products of combustion; but the foregoing are the chief, viewed as the results of an explosion, since upon these the force developed almost wholly depends.
Force developed by an Explosion.
—A consideration of the facts enunciated in the foregoing paragraphs will show to what the tremendous energy developed by an explosion is due. It was pointed out that the combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If this volume of gas be compressed within the space of 1 cubic foot it will obviously have a tension of 31·5 atmospheres; that is, it will exert upon the walls of the containing vessel a pressure of 472 lb. to the square inch. If the same volume be compressed into a space one-eighth of a cubic foot in extent, say a vessel of cubical form and 6 inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen to exist in the solid state, and the two bodies carbon and oxygen to occupy together a space of one-eighth of a cubic foot, the combustion of the carbon will develop upon the walls of an unyielding containing vessel of that capacity a pressure of 252 atmospheres. Also the combustion of 1 lb. of hydrogen gives rise, as already remarked, to 190·26 cubic feet of steam; and if combustion take place under similar conditions with respect to space, the pressure exerted upon the containing vessel will be 22,830 lb., or nearly 10·5 tons, to the square inch, the tension being 190·26 × 8 = 1522 atmospheres.
The force thus developed is due wholly to the volume of the gas generated, and by no means represents the total amount developed by the explosion. The volume of the gases evolved by an explosion is estimated for a temperature of 62°; but it was shown in a former paragraph that the temperature of the products of combustion at the moment of their generation is far above this. Now it is a well-known law of thermo-dynamics that, the volume remaining the same, the pressure of a gas will vary directly as the temperature; that is, when the temperature is doubled, the pressure is also doubled. By temperature is understood the number of degrees measured by Fahrenheit’s scale on a perfect gas thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale, that is, 493°·2 below the freezing point of water. Thus the temperature of 62° for which the volume has been estimated is equal to 461·2 + 62 = 523°·2 absolute.
It was shown that the temperature of the product of combustion when carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent to 10179°·2 absolute. Hence it will be observed that the temperature has been increased 10179°·2 523°·2 = 19·45 times. According to the law above enunciated, therefore, the pressure will be increased in a like ratio, that is, it will be, for the volume and the space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square inch.
When carbon is burned to carbonic acid, the temperature of the product was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute. In this case, it will be observed that the temperature has been increased 23977·2 523·2 = 45·83 times. Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3 tons to the square inch. It will be seen from these pressures that when combustion is complete, the force developed is 2·36 greater than when combustion is incomplete; and also that the increase of force is due to the larger quantity of heat liberated, since the volume of the gases is the same in both cases. If we suppose the carbon burned to carbonic oxide in the presence of a sufficient quantity of oxygen to make carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If this volume be compressed within the space of one-eighth of a cubic foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will exert upon the walls of the containing vessel a pressure of 5124 lb. to the square inch. The temperature of the gases will be 32 + 4400 0·190 × 3·667 = 6347° Fahr. = 6808°·2 absolute, the mean specific heat of the gases being 0·190; whence it will be seen that the temperature has been increased 6808°·2 523·2 = 13·01 times. According to the law of thermo-dynamics, therefore, the pressure under the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons to square inch. So that, under the conditions assumed in this case, the pressures developed by incomplete and by complete combustion are as 29·8 to 77·3, or as 1 to 2·59.
Similarly, when hydrogen is burned to water, the temperature of the product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2 absolute; and the pressure will be 22,830 × 16510·2 523·2 = 720,286 lb. = 321·1 tons to the square inch.
It will be observed, from a consideration of the foregoing facts, that a very large proportion of the force developed by an explosion is due to the heat liberated by the chemical reactions which take place. And hence it will plainly appear that, in the practical application of explosive agents to rock blasting, care should be taken to avoid a loss of the heat upon which the effects of the explosion manifestly so largely depend.