DIFFRACTION. 236 DIFFUSION. bino tlioui >vitli telescopes iinJ coUiiiiatois: but Piolcssor Kowland has shown lliat rellectiiig gratings ruled on spherically concave surfaces can be used without the addition of lenses. These "concave-gratings' have other marked advantages over plane ones for almost every s|>ectroscopic purpose. A good description of their properties is given in Preston's Theory of Light (London, 1895). DIFFUSION (l.at. diffiisio, a spreading out, from (liffuiHlcrc, to difl'use. from dis-, apart + fiiiKlvrc, to pour). The gradual dispersion of the particles of one substance among those of an- other. DlFFl'Slo.N OF LiiiViDS. If sulphuric acid be carefully poured through a tube into the bottom of a vessel filled with water colored by a few drops of litmus solution, and the liquids allowed to remain undisturbed, the .acid will be seen gradually to dilVuse upward into the water, 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 water will have completely intermixed, form- ing a dilute solution of sulphuric acid of uniform strength throughout. A similar process would take jilace in llio 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 cases, however, tlic rale of dilFusion would be so small that it might take months and even years before the solutions would be- come perfectly homogeneous. The reason of this is that the surface of separation of two liquids is, under ordinary circumstances, comi>aratively very small. To render the diffusion more rai)id. the surface through which diffusion takes place would liave to be increased: and this is usually attained by slirriiui the two liquids with the aid of some soliil object, 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- cules. DiFFisiox OF Gases. If two flasks be filled, one with hydrogen, the other with chlorine, and connected by a long tube fitted into their necks Ipy means of corks, then, in whatever jiosition the apparatus be placed, it will he found that the gases iniitually interpenetrate. The color of chlorine will in this case enable us to follow by the eye the course of the diffusion. When the mixture has attained its permanent state, each of the gases is found to be uniformly diffused through the whole containing space, precisely as it would have been had the other not been present. In fact, the presence of a second gas seems merely to affect the time which (he first takes to distribute itself evenly througlunit the vessel, but in no other Avay to influence the final result. Dalton long ago suggested the analogy of the passage of water among stones in the bed of a river. The molecules of gases are capable of passing through solid partitions, which would prevent the motion of masses. The iihenomena of dif- fusion can therefore bo readily investigated by measuring the amounts of various gases parsing in a given interval of time through a thin layer of bladder or other membrane, or through a thin disk of planter of Paris, graphite, or biscuit-ware. The vehx-itv of diffusion is thus found to be in- versely )iroi)ortioual to the density of the gaa c.periuieMled ui>oii. That lighter ga>es ditTu^e more rajiidly 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 at one end with a diaphragm of plaster of Paris and filled with hydrogen. Its open end is innnersed in water, care being taken not to wet the diaphragm. Two jirwcsses of dill'usion will then take place sinuiltaneously: the hydrogen will diffuse out of the tube into the atmo>phere, while the constituent gases of the air will pass through the diai>lirngm inward. But as hydro- gen is much lighter than the gases of the air, the velocity with which it escapes is gieater 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 decrea.<€ until all the hydrogen has dilTused out. See, also, Kffisio.n. DiFFfsioN of Substances in Solution. Ac- cording to modern chemical theory, solid sub- stances in solution exhibit the same phenomena as substances in the gaseous state. Within a given volume of solution, a substance exercises, for instance, precisely the same pressure (osmotic pressure) that it would exercise if vaporized and confined within a vessel whose volume is equal to the volume of the solution. This explains the perfect analog^' existing bctwiH'U the ]>licnomena of diffusion of gases and the phenomena of dif- fusion of substances in solution. Diffusion in solution was discovered in 1815 by Parrot, and was later (about 1850) made the subject of a thorough investigation by Thomas Craham. Shortly -afterwards Fick suc- ceeded in proving, both theoretically and experi- mentally, that the diffusion of dissolved sub- stances follows ])recisely the same law as the dif- fusion of heal ( Fourier's law). Accordingly, the amount of substance traversing a small (listancc within the solvent is proportional to the area of the cross-section throvifih which the dillusion takes place, and to the difference in concentra- tion at the two ends of the small distance tra- versed. The rate of ditTusion in a given solvent depends, of course, also upon the nature of the diffusing substances, and upon the temperature; at higher temperatures, the rate of diffusion is considerably greater than at lower ones. In more recent years Xcrnst deduced these laws from the theory of osmotic pressire. The fact that sugar, for instance, diffuses in solution nni<h more rapidly than a gas whose pressure in different parts of a vessel has become unequal, is explained by the fact that friction in liquids is much greater than friction in gases. The laws of osmotic pressure permit of calculat- ing the exact amount of resistance offered by the friction of the solvent to the iliffusion of the dissolved sidistance. if the rate of diffusion haa been determined experimentally. The resistanc* is found to be enormously great. Thus, in thfl ease of sugar difftising in aqueous solution, the force required to move .'?4'2 grams at (he rate of one centimeter a second, at the temix-rature of 18° Centigrade, is equal to the weight of 4,700.- 000.000 kilograms. 'riie components of substances which exist in xdutiiins in a state of chemical dissiK-iatii>n may be separated by diffusion, for each of those compounds has its own iharacterisdc ra(e of dif- fusion. In this manner, for instance, at least
