As a point of historical interest, we may mention that many years before the publi• cation of Faraday's earliest researches on this subject, sulphurous acid gas had been liquefied by Monge and Clouet, ammonia by Guyton 3lorveau, and arseniureted hydro gen by Stromeyer, by the simple application of cold, without any increased pressure.
The expansion and coutruction of gases by changes of temperature is treated of under HEAT.
The process of intermixture iu gases, and the movements of these substances gen erally, have been yen/ carefully studied by Faraday, Dobereiner, Mitchell, Bunsen, and especially Graham. 'These movements are usually considered under four heads, viz.: 1. Difusion, or the intermixture of one gas with another; 2. Effusion, or the escape of a gas minute aperture in a thin plate into a vacuum; 3. Transpiration, or the passage of different gases through long capillary tubes into a rarefied atmosphere; 4. Oonosis, or the passage of gases through diaphragms.
In the article _Diffusion (q.v.), the general principles of this kind of movement in gases are sufficiently explained, and we shall merely make one or two supplementary remarks, chiefly with the view of rendering the following table more intelligible. Graham's experiments with the simple diffusion-tube show (see Graham's memoirs in the Trans actions of the Royal Societies of London and Edinburgh, or Miller's Chemical Physics) that the diffusiveness or diffuiion volume of a gas is in the inverse ratio of the square root of its density; consequently, the squares of the times of equal diffusion of the different gases are in the ratio of their specific gravities. Thus, the density of air taken as the standard of comparison at 1, the square root of that density is 1, and its diffusion volume is also 1; the density of hydrogen is 0.0692, the square root of that density is 0.2632, and its diffusion volume is , or 3.7904; or, as actual experiment shows, 3.83—that is to say. if hydrogen and common air be placed under cireunistances favor ing their mutual diffusion, 3.83 volumes of hydrogen will change place with 1.00 of air. The following table gives: 1. The density; 2. The square root of the density; 3. The calculated, and 4. The observed velocity of diffusion or diffusiveness of several impor tant gases; the numbers in the last column, headed " Rate of Effusion, "being the results obtained by experiment upon the rapidity with which the different gases escape into a vacuum through a minute aperture about of an inch in diameter.
" The process of diffusion," says prof. Miller, " is one which is continually perform ing an important part in the atmosphere around us. Accumulations of gases which are unfit for the support of animal and vegetable life are by its means silently and speedily dispersed, and this process thereby contributes largely to maintain that uniformity in the composition of the atirial ocean which is so essential to the comfort and health of the animal creation. Respiration itself, but for the process of diffusion, would fail of its appointed end, in rapidly renewing to the lungs a fresh supply of air, in place of that which has been rendered unfit for the support of life by the chemical changes which it has undergone." A reference to the last two columns of the above table shows that, within the limits of experimental errors, the rate of effusion of each gas coincides with its rate of dif fusion.
Graham's experiments show that theyelocity'of transpiration (the term which that chemist applied to the passage of gas through long capillary tubes) is entirely independ ent of the rate. of diffusion, or of any other known property. It varles.with the chemi cal 'nature of the gas, and is most probably " the resultant of a kind of depending upon the absolute quantity of heat, latent as well as sensible, which different gases contain under the same volume; and therefore will be found to be connected more immediately with the specific heat than with any other property of gales." Oxygen is found to have the lowest rate of transpiration. Taking its transpiration velocity atV, that of air is 1.1074; of nitrogen, 1.141; of carbonic acid, 1.'869; of sulphureted hydro gen, 1.614; of ammonia, 1.935; of olefiaut gas, 1.980; and 2.288.
In the passage of gases through diaphragms, the law of the diffusion of gases is more or less disturbed or modified according to the force of adhesion in the material of which the diaphragm is composed; the disturbance being greatest in the case of soluble gases and a moist thin diaphragm, such as'a bladder or a rabbit's stomach. Fo/details on this subject we must, however, refer to the article OSMOSE.