In Mr. Graham’s experiments relating to effusion, a gas under a constant pressure was on one side of a minute opening in a very thin plate, and a vacuum on the other. The rapidity with which air or gases enter the vacuum depends upon their specific gravity. A gas rushes into a vacuum with the speed acquired by a heavy body in falling from the height of an atmosphere of the gas in question supposed to be everywhere of the same density. The height of this uniform atmosphere will be in an inverse ratio to the density of the gas. An atmosphere of hydrogen, for example, will be 16 times higher than one of oxygen. But the velocity acquired by a heavy body not being in direct proportion to the height, but to the square root of the height, it follows that the rate of flow of different gases into a vacuum will be in an inverse ratio to the square root of their respective densities. The rate of flow of oxygen being represented by 1, that of hydrogen will be represented by 4 the square root of 16. This law has been verified by experiment, and is quite analogous to that which regulates molecular diffusion, but the phenomena are essentially different. It is the gas en masse which partakes of the movements of effusion, whilst only the molecules or atoms of a gas are affected by the movements of diffusion. For that reason the swiftness of the effusion of a gas is many thousand times greater than that of diffusion. The swiftness of the efflux of atmospheric air is as rapid as the velocity of sound.
The rate of the flow of different gases under constant pressure through capillary tubes into a vacuum, constitutes the capillary transpiration of gases. These rates bear a constant proportion to one another, but they are singularly unlike the rates of effusion. They are independent of the material of the tube; they are not governed by specific gravity; and ‘they appear to be in constant relation with no other known property of the same gases; and they form a class of phenomena remarkably isolated from all else at present known of gases.’
The pores of graphite are so fine that it is incapable either of effusion or transpiration, but it is readily penetrated by means of the molecular or diffusive movements of gases, as appears on comparing the time requisite for the passage of equal volumes of different gases under constant pressure into a vacuum. For oxygen, hydrogen and carbonic acid gas, the times are nearly as the square roots of their densities.
The atmolysis or partial separation of mixed gases and vapours of unequal diffusibility, can be effected by allowing the mixture to penetrate through a graphite plate into a vacuum. The amount of separation is in proportion to the pressure, and attains its maximum when the gases pass into a perfect vacuum. One of the results of atmolysis was the concentration of oxygen in atmospheric air. When a portion of air confined in a vessel was allowed to penetrate into a vacuum through graphite or unglazed earthenware, the nitrogen passed more rapidly than the oxygen in the ratio of 1·0668 to 1, and the portion of oxygen is proportionally increased in the air left behind in the vessel. The increase of oxygen actually observed when the air in the vessel was reduced from 1 volume to 0·5 was 0·48 per cent. The diffusion was continued till the air in the vessel was reduced to 0·0625 and the concentration of the oxygen in it amounted to 2·02 per cent. The molecular or diffusive mobility exercises a certain influence on the heating of gases by contact with heated liquid or solid substances. The more rapid the molecular movement of a gas is, the more frequent will be the contact of the molecules and the quicker will be the communication of heat. The greater cooling power of hydrogen compared with that of oxygen or air is probably owing to that cause. ‘Oxygen and hydrogen gas have the same specific heat for equal volumes; but a hot object placed in hydrogen is really touched 3·8 times more frequently than it would be if placed in oxygen gas. Dalton had already ascribed this peculiarity of hydrogen to the high mobility of the gas.’[[13]]
It appears that isomorphic substances such as chloride, bromide, and iodide of sodium, have a similar diffusibility, another of the many analogies between these singular marine substances.
Modem chemistry is essentially experimental; the unprecedented magnitude to which British manufactures have risen is chiefly owing to experiments conducted with consummate skill and dexterity. In these investigations, accidental circumstances have sometimes occurred which led to other researches quite different from that originally in view, which have had unexpected and invaluable results. Although the simple elements are few, they are capable of an infinite variety of combinations, so that by analysis and new combinations, the most useful and valuable materials are now obtained from obnoxious or useless substances, formerly thrown away. The instances are numerous; but sawdust may be mentioned as one of the most remarkable. It was not even fit for fuel, but now oxalic acid, a bleaching principle most extensively used in the various processes of calico printing, is procured from it; the quantity required may be imagined, since the cotton cloth annually printed in Great Britain previous to the American war, would surround the earth’s equator nineteen times. Oxalic acid, which is a vegetable substance, found combined with potash in wood sorrel or Oxalis acetosella, used to be made from sugar or starch, by the action of nitric acid. Now starch, sugar, and woody fibre or fibrine, all contain twelve parts of carbon and different portions of oxygen and hydrogen, always in the proportions that form water; hence the name of carbohydrates. Their composition is so similar that the one may be changed into the other by the addition or subtraction of one or two atoms of water under its atomic form; thus when fruits ripen, the starch they contain is changed into sugar by the addition of one atom of water under its dry form.
Now sawdust is woody fibre, and might be changed by nitric acid into oxalic acid like the others. But a less expensive method is actually employed.
When sawdust, mixed with two equivalents of the hydrate of soda and one equivalent of the hydrate of potash, is exposed to a heat of 400° for a few hours, the substances are fused, and when raised to a still higher temperature the hydrates are decomposed: hydrogen is evolved, and the carbon combines with the oxygen to form the oxalate of soda and the oxalate of potash. In order to separate these oxalates they are put into a filter, a solution of carbonate of soda is passed through it; the oxalate of soda remains in the filter, the carbonate of potash passes through it; and when lime is added to the oxalate of soda, the soda is liberated, passes through the filter, and the oxalate of lime remains. Sulphuric acid is then added to the oxalate of lime, sulphate of lime is formed, and oxalic acid mixed with water remains, and by evaporation forms into beautiful crystals of oxalic acid. This is an instance of a complicated chemical process; nevertheless it is carried on to a vast extent in Manchester, nine tons a week being furnished by one manufactory alone. Two pounds of sawdust yield one pound of oxalic acid.
In ordinary distillation a volatile substance such as water, by absorbing the heat applied to it, becomes converted into vapour; by abstracting the absorbed heat from the vapour, it is reconverted into the original substance. Destructive distillation, on the contrary, consists of an entire destruction of the original substance and a simultaneous production of new substances. Of this the destructive distillation of coal furnishes the most interesting illustration, and shows at the same time the success of modern chemistry in utilizing waste substances.
Coal had been distilled for years to furnish gas for the illumination of our cities before it was discovered that the refuse contained principles of the greatest value. The products of the distillation are threefold: gas, coal water, and coal tar.