Page:The American Cyclopædia (1879) Volume VII.djvu/646

 634 GAS osophical Magazine." The same result was obtained by Berthollet with a tube 10 inch- es long and one fifth of an inch in calibre, when the apparatus was placed in a position which secured a uniform temperature, so that no motion could be communicated to the gases. When the upper vessel contained hydrogen, the time occupied in diffusion was about 12 days ; but when it contained air, oxygen, or nitrogen, several weeks were occupied in the complete diffusion. If a cylinder is filled with any gas and placed in a horizontal position, and an open tube bent at right angles is in- serted at one end, turned upward if the gas is lighter than air, and downward if heavier, after a time the gas will escape from the cylin- der and its place be occupied by air. The re- sults given in the following table were ob- tained by Graham, and show the proportions of different gases which escaped from 100 vol- umes in four and in ten hours : NAME OF GAS. Sp.gr. In4h. In 10 h. 1 81-6 94-5 Light carburetted hydrogen Ammonia 8 8*5 43-4 41*4 62-7 59'6 defiant gas 14 84'9 48'3 Carbonic acid 22 81'6 47-0 Sulphurous acid 82 27-6 46-0 85*4 23 -7 89 '5 It is here seen that the lighter gases are the more readily they escape, and that this ten- dency to diffusion is nearly in the inverse pro- portion of the square root of their densities. If the cylinder contains a mixture of gases, those which are the lightest and therefore the most readily diffusible will escape with the greatest rapidity. Vapors also diffuse themselves among one another and among the permanent gases in accordance with the same law. If the mouth of the tube, hi the apparatus mentioned above, is closed with a porous substance like plaster of Paris or wood, and the cylinder is filled with hydrogen, this gas will escape much faster than the air will enter. It appears, therefore, that the smaller the calibre of the orifices through which the diffusion takes place, the greater will be the proportional rapidity of the transfusion of the lighter gas ; a fact having a close relation to the phe- nomena of osmose, which it g*eatly aids in explaining, raham has made extensive experiments on the diffusion of gases. By using an instru- ment called a diffusion tube or diffusiometer, by means of which exact measurement could be made of the rate at which interchange took place, he found that diffusion through porous septa follow- ed the same law as when the communication was by tubes of sensible diam- eter ; that is, that the diffusibility of gases is in FIG. 5. Graham's Diffusiometer. proportion to the square roots of their densities. The diffusiometer used by Graham (fig. 5) con- sists of a glass tube of about one inch calibre and one foot in length, and a vessel partly filled with mercury. One end of the tube is stopped with a plug of plaster of Paris, one fifth of an inch thick, which is formed by mix- ing the plaster into a paste with water, intro- ducing it while in a plastic state, and allowing it to set and dry. When the tube is filled with hydrogen and its open end placed in the vessel of mercury, diffusion takes place through the porous plaster plug, the atmospher- ic air passing in and the hydrogen passing out ; but the latter action is so much the more rapid that in three minutes the mercury will rise in the tube three inches above its level in the outer vessel, and in 20 minutes all the hydro- gen will escape. Graham afterward used com- pressed graphite, such as is used in making writing pencils, in place of the plaster of Paris, and considered it superior; but the results which he obtained with it did not alter the conclusions arrived at in the first series of ex- periments. Atmolysis, or the Separation of Gases ~by Diffusion. When a mixture of gases passes through a graphite plate into a vacuum, a rapid separation takes place, in proportion to the difference of pressure. This method of separation may be performed with an apparatus called an atmolyser (fig. 6). A porous earth- FIG. 6. Atmolyser. enware pipe, &, passes from a bag, &, containing the mixed gases, to the receiver c, over a pneu- matic cistern. A large tube, e, surrounds the smaller one, and the space between them is kept as nearly vacuous as possible by means of an air pump exhausting through the tube d, A slight pressure upon the bag causes the gases to flow through the tube a, but the lighter ones will to a great extent pass through the sides of the tube and be removed by the air pumps. In an experiment made in this man- ner with atmospheric air, the proportion of oxygen was increased to 24*5 per cent. When a mixture of oxygen and hydrogen is used, the separation is much greater. In one experiment an explosive mixture of one volume of oxygen with two of hydrogen, after atmolysis, con- tained only 9-3 per cent, of hydrogen, in which a taper burned without producing explosion. The diffusion of gases through one another was regarded by Dalton as a necessary consequence of the self-repulsive property of the particles of gaseous bodies, by reason of which each gas expands into the space occupied by the other as it would into a vacuum. But although the velocities with which they diffuse into each other are in proportion to those with which