104 SOLUTION OF GASES. [63. in nitric acid with (2 HNO3, 3 H,O), and in alcohol with the hydrate (C,HO, 3 H2O). (64) Adhesion of Gases to Liquids.-The adhesion of gases to liquids, although not quite so evident as that of solids to liquids, is yet attended with results almost equally important. It is exemplified in the pouring of liquids from one vessel to another, by the bubbles which are carried down with the descending stream, and which rise and break upon the surface of the liquid. Adhesion, however, produces in the effects of solution which attend the mutual action of gases and liquids, results which are far more general in their operation. All gaseous bodies are in a greater or less degree soluble in water: some, as hydrochloric acid and ammonia, being absorbed by it with extreme rapidity, the liquid taking up 400 or 600 times its volume of the gas; in other instances, as occurs with carbonic anhydride, water takes up a volume equal to its own; whilst in the case of nitrogen, oxygen, and hydrogen, it does not take up much more than from a twentieth to a fiftieth of its bulk. As the elasticity of the gas is the power which is here opposed to adhesion, and which at length limits the quantity dissolved, it is found that the solubility of each gas is greater, the lower the temperature, and the greater the pressure exerted upon the surface of the liquid. Dr. Henry found that at any given temperature, the volume of any gas which was absorbed was uniform, whatever might be the pressure; consequently that the weight of any given gas absorbed by a given volume of any liquid at a fixed temperature increased directly with the pressure. If the pressure be uniform, the quantity of any given gas absorbed by a given liquid is also uniform for each temperature; and the numerical expression of the solubility of each gas in such liquids is termed its coefficient of absorption, or of solubility, at the particular temperature or pressure; the volume of the gas absorbed being in all cases calculated for o° C., under a pressure of 760mm. of mercury. For instance, I volume of water, at 0° C., and under a barometric pressure of 760mm., dissolves 0'04114 of its volume of oxygen; and this fraction represents the coefficient of absorption of oxygen at that temperature and pressure. All water contains a certain small proportion of air in solution, in consequence of the solubility of the gases of which the atmosphere consists; and if placed in a vessel under the airpump, so as to remove the atmospheric pressure from its surface, the dissolved gases rise through the liquid in minute bubbles. Small as is the quantity of oxygen thus taken up by water from 64.] ABSORPTION OF GASES BY LIQUIDS. 105 the atmosphere, it is the means of maintaining the life of all aquatic plants and animals; if the air be expelled from water by boiling, and it be covered with a layer of oil to prevent it from again absorbing air, fish or any aquatic animals placed in such water quickly perish. Even the life of the superior animals is dependent upon the solubility of oxygen in the fluid which moistens the air-tubes of the lungs, in consequence of which this gas is absorbed into the mass of the blood as it circulates through the pulmonary vessels. If a mixture of two or more gases be placed in contact with a liquid, a portion of each gas will be dissolved, and the amount of each so dissolved will be proportional to the relative volume of each gas in the mixture multiplied into its coefficient of solubility at the observed temperature and pressure :-For instance, if it be assumed in round numbers, that atmospheric air contains one-fifth of its bulk of oxygen, and four-fifths of its bulk of nitrogen, the amount of each of these gases which water should absorb from the air at a temperature of 15° C. under a pressure of 760mm, may be calculated in the following manner. The coefficient of absorption for oxygen at 15° C. is 002989, that of nitrogen is o'01478: = 002989 000597 proportion of oxygen dissolved. 001779 proportion of air dissolved. The proportion of nitrogen thus required by calculation is rather less than double that of the oxygen, or 66'1:33'9, a proportion which agrees almost exactly with the results of experiment. When water saturated with one gas is brought into contact with another gas, the second gas will gradually expel the first, and if the quantity of this new gas be very large, it may displace the first altogether. Hence it is impossible to preserve gases in a pure condition over water. Every gas is to a certain extent soluble in water, and this solution is perpetually giving out its gas into the atmosphere, and absorbing oxygen and nitrogen from the air, so that these gases become diffused slowly into the jar which confines the gas under experiment. A jar of hydrogen confined over water in the pneumatic trough will thus in a few days become contaminated with atmospheric air, although it may have experienced little apparent change in bulk. From the experiments of Bunsen, of Roscoe, and others, it appears that Henry's law is exact for the gases of moderate solubility, but the more soluble gases, such as ammonia, hydrochloric 106 ADHESION OF GASES TO SOLIDS. [64. acid, and sulphurous anhydride, except under moderate pressure, and at mean temperatures, do not strictly follow the law. In these instances disturbing causes are evidently at work. In the case of chlorine, for example, at low temperatures a definite crystalline hydrate is formed, so that true chemical attraction conspires to exalt the effect of adhesion. In the solution of the very soluble gases a considerable elevation of temperature occurs, and a great increase in the bulk of the liquid takes place. The following table shows the solubility of some of the principal gases, both in water and in alcohol, with the barometer at 760mm. (Bunsen, Liebig's Annal. 1855, xciii. 1, and Carius, Ib. 1855, xciv. 129.) All these gases, with the exception of hydrochloric acid, may be expelled from the water by long-continued boiling : Solubility of Gases in Water and in Alcohol. Other liquids besides water and alcohol dissolve the gases with greater or less avidity. (65) Adhesion of Gases to Solids.-When iron filings are gently dusted over the surface of a vessel of water, a considerable body of iron dust may be accumulated upon the surface, until at length it falls in large flakes, carrying down with it bubbles of air of considerable size. The adhesion of these bubbles caused the particles of iron to float, for such particles are nearly eight times as heavy as water. Contrasted with this result is the effect of dusting magnesia in fine powder over the surface of water; the 65.] ABSORPTION OF GASES BY CHARCOAL. 107 particles, although not one-third of the density of the iron, immediately become moistened and sink. In consequence of this adhesion of air to their surface, many small insects are enabled to skim lightly over the surface of water, which does not wet them. If a slip of clean platinum be placed in mercury, it is found on withdrawing it to come out dry, but if the mercury be boiled on the platinum, the film of air which separated the two metals is expelled, and the mercury will be found to have wetted the surface completely. It is this adhesion of air to the surface of glass which renders it necessary, in making barometers, to boil the mercury in the tubes after they have been filled, in order completely to expel the film of air with which the tube is lined. But the most striking instances of adhesion between gases and solids are exhibited when finely-divided bodies are made the subject of experiment. We have already had occasion to notice the effect of charcoal when introduced into solutions (54). Its effects on gases are equally remarkable. If a piece of well-burnt boxwood charcoal be plunged whilst red-hot under mercury, and introduced without exposure to the air into a jar of ammonia or of hydrochloric acid, it will absorb these gases with great rapidity, and will indeed reduce them into a bulk less than that which they would occupy in the liquid form. A piece of freshly-burned charcoal when exposed to the air condenses moisture rapidly within its pores, and has been observed from this cause to increase in weight nearly one-fifth in a few days. Owing to this property of charcoal, water saturated with many gases may be freed from them when filtered through a body of ivory black: sulphuretted hydrogen may thus be removed so completely, that it cannot be detected either by its nauseous odour, or by the ordinary tests. De Saussure (Thomson's Annals of Philosophy, 1815, vi. 243) found that freshly-burned boxwood charcoal absorbed different gases in very different proportions, as will be seen in the following tabular view of his results, where the bulk of the charcoal used in each experiment is taken as 1: It will be seen that these results follow an order almost exactly the same as that of the solubility of the gases in water (64). 108 ABSORPTION OF GASES BY CHARCOAL. [65. Different kinds of charcoal vary considerably in this absorbent power. The denser the charcoal the greater is its absorbent power. Hunter found that formed from the shell of the cocoanut was the most powerful absorbent of any which he tried, both ebony and logwood charcoal being much inferior. Stenhouse found, on comparing equal weights of three different forms of charcoal, that the relative absorbent power of each was as follows: In these experiments, o'5 gramme of each kind of charcoal was employed, and the numbers in the table indicate, in cubic centimetres, the quantity of each gas absorbed. The more recent experiments of Hunter and Angus Smith confirm these results. This absorbent action of charcoal renders it a valuable agent in the purification of atmospheric air when contaminated with offensive products of decay or putrefaction, such air being, as Stenhouse has shown, rendered inodorous by simply passing it over fragments of wood charcoal, a true oxidation of many of these bodies being effected in the pores of the charcoal (366). Charcoal which is saturated with one gas, if put into a different gas, gives up a portion of that which it had first absorbed, and takes up in its place a quantity of the second. Finely divided metallic platinum also condenses in its pores a large quantity of many gases, amounting in the case of oxygen to very many times its own volume. If a jet of hydrogen gas be allowed to fall in the open air upon a ball of spongy platinum, or platinum in a fine state of subdivision, the metal becomes incandescent; the oxygen and hydrogen combine rapidly within the pores of the metal, and the heat given out usually sets fire to the jet of hydrogen: ether and alcohol, when dropped upon platinum black, another still more finely-divided form of the metal, produce a similar appearance of incandescence. This property of platinum is turned to account in effecting many important chemical changes. |