DIFFUSION OF Lithium. If sulphuric acid be carefully poured through a tube into the bottom t a vessel tilled with water colored by a few drops of litmus solution, and the liquid: allowed to remain undisturbed, the acid Will he seen gradually to lh upward into the waiter, its progress being indicated by the change of color of the litmus from blue to red. After a suffi ciently long time the liquid will be found per fectly homogeneous: i. e. the two layers of acid and \voter ycill have completely intermixed, form ing a dilute solution of sulphuric acid of uniform strength throughout. A similar process would take place in the case of water and alcohol, and, in general, in the case of any pair of liquids that are at all capable of being mixed together. in many eases, however, the rule of diffusion would be so small that it might take months and even years before the solutions world be come perfectly homogeneous. The reason of this is that the surface of separation of two liquids is. under ordinary circumstances, comparatively very small. To render the diffusion more rapid, the surface through which takes place would have to lie increased: and this is usually attained by stirring the two liquids with the aid of some solid .tbiect, as a glass rod or a spoon. Diffusion itself, however, is a purely molecular process; that is to say, it involves the motion not of masses of liquid, but of mole cults.
Dtrrt•stos or axsrs. If two flasks be tilled, ne with hydrogen, the other with chlorine. and connected by a long tube fitted into their by means of corks, then. in whatever position the apparatus be placed, it will lie found that the gases mutually interpenetrate. The color of chlorine will in this ease unable us to follow by the eye the course of the diffusion. When the mixtiire has attained its permanent state. each
of the gases is found to be uniformly diffilsed through the whole containing space, precisely a- it would have been had the other not been present. In fact, the presence of a second gas ...ems merely to affect the time which the first takes to distribute itself evenly throughout the Nit in no other way to influence the tinal result. Dalton long ago suggested the analogy of the passage of water among stones in the Led of a river.
The molecules of gases are capable of through solid partitions, which \\'0111•l prevent tl e motion of The 11111.110111ella of dif fusion therefore he readily investigated by measuring the amounts of various gases passing in a given intenal of time a thin layer of bladder or other membrane. or through a thin disk of plaster f Paris, graphite. or Ids/lilt-ware. The velocity of Is to b.• xersely proportional to the density of the gas experimented upon. 'Ilitit lighter gases diffuse more rapidly than heavier ones may be demon strated as follows: A glass tube, say one inch in diameter, and two or three feet long, is closed :It one end with it diaphragm of plaster of Paris and tilled with hydrogen. Its open end is immersed in water, care being taken not to wet the diaphragm. Two processes of diffusion will then take place simultaneously: the hydrogen will diffuse out of the tube into the atmosphere, while the constituent gases of the air will pass through the diaphragm inward. But as hydro gen is nitwit lighter than the ga-cs of the air, the %clocity with it escapes is greater than that with which the others enter. this being shown by a rise of water in the tube, whose part occupied by gas will continue to decrease until all the hydrogen has diffused out. See, also,