Bunsen has suggested the application of the method of diffusion to the very impor tant question in gas analysis—whether the constituents of a gas, as determined by the usual methods, are merely mixed, or are chemically united. It is evident that, in general, the diffusion rate of a mixture of two gases will differ from that of a compound of the same.
2. Diffusion of Saline Matters in Solution.—If a strong brine be placed in the bottom of a tall glass jar, pure water may be carefully introduced above it, so that no immediate mixture takes place. If the whole be allowed to stand, the salt is gradually diffused through the vessel, which, after a sufficient time, will be found to contain a brine of uniform strength. Experiments have been carefully made to determine, in such a case as the above, the distribution of the salt through the vessel at various periods before the permanent state has been arrived at. They have been compared with the results of the theory now to be explained, and the coincidence has been found very satisfactory. The theory assumes that the rate of diffusion between contiguous layers of the water in the cylinder is proportional to the excess of salt in one layer above that in the next—the co-efficient of proportionality' involving a special constant of diffusion for the particular salt experimented upon. This is precisely the assumption that is made the linear conduction of heat in a homogenous solid, or the propagation of electricity in a wire.
drc dtu The partial differential equation to which all of these cases are reducible, (C— = ' was obtained, and its complete solution exhibited in various forms long ago by Fourier, in his de la Chalettr, one of the most remarkable mathematical investigations of last generation. See HEAT, CONDUCTION OF, and ELECTRICITY, THEORY OF. It is curious to consider the heating of a metallic rod, or the solution of a few crystals of salt in a tall glass jar full of water, as problems thus directly allied to the signaling through the Atlantic cable.
Graham's method of determining the diffusion co-efficient of a salt in water was sim ple, and yet admitted of great precision in the determinations. A number of glass bot tles, cast in the same mold, had their mouths ground flat, so as to lie accurately closed by a plate of glass, which—when the bottle, filled with a solution of known strength, had been carefully placed in one of a series of equal glass jars, and covered with a constant amount of water—could be slipped off without producing any considerable disturbance in the fluid. After a measured time, the glass plate was replaced, and the amount of salt which had loft the botWitCcurithy determined.
The following are the most important of the laWs thus obtained; they are quite con sistent with the theory above mentioned. For solutions of the same substance, of dif7 ferent degrees of strength, the rate of diffusion is proportional to the strength of the solution. Different salts seem to arrange themselves in groups as regards their diffusion co-efficients, the latter having simple numerical relations to each other. Analogy of chemical composition and of crystalline form appear to be the principal elements in the arrangement of the groups. The quantity diffused increases with the temperature, and at the same rate for all salts. The presence of a second salt in the solution, or in the water into which the diffusion takes place, if not in large quantity, appears not to affect the result, supposing, of course, that no chemical action takes place. It is evident that
by this process a partial separation may be effected of salts which have different rates of diffusion, and do not act chemically on each other; and it is found that in certain cases even chemical compounds, such as alum, may be partially decomposed by the same means.
3. Diffusion of Liquids. sulphuric acid be carefully poured through a tube into the bottom of a vessel filled with water, colored by an infusion of litmus, or red cabbage, the change of color of the vegetable dye will enable us to trace the gradual diffusion of the acid in the water. Here, the process, though probably on the whole quite analogous to the case of gases, occupies more time; but the final result is. as in the former case, an almost uniform mixture of the two fluids. There is no necessity for any special remark on this part of the subject, particularly as we have already adverted to the theory of this process. But if different fluids be separated by a mem brane or diaphragm, some extremely remarkable results are obtained, which were first carefully examined by Dutrochet. These have been attributed to the action of osmotic force, something of the same kind as capillary force, and probably a closely connected, if not identical form of molecular action. The theory of these actions is not yet well understood, but we shall endeavor to give from analogy a few attempts at explanation.
If an inverted funnel, with a very long stem, have a bladder tied over its mouth, and, being filled to the neck with sirup of sugar, be suspended so that the bladder is entirely under the surface of water in a dish, the sirup will pass through the bladder into the water, and the water will pass through in the opposite direction, but in far greater quantity—producing the extraordinary effect of a rise in the level of the fluid in the tube, which can with precaution be made to amount to a yard or two in the course of it few days. The points of the attempted explanation of this phenomenon are some what as follows: The bladder has more capacity for, or will absorb more of, water than of sirup. The first effect, then, is to saturate the bladder with water, very slightly mixed with sirup, On the lower side of the bladder, water, with a small quantity of sugar in solution, is diffusing into pure, or nearly pure water, this process will be a slow one; at the upper side, water (nearly pure) is diffusing into a strong sirup. Here, then, the effect is much greater, and thus a greater quantity of water passes upwards than of sirup downwards. Similar effects may be produced with a vast number of other liquids. Combined with capillarity, it is believed that these experiments explain the motion of the sap in vegetables, and various other phenomena in the vegetable and animal kingdoms.
DIGABIlIA, an obsolete letter of the Greek alphabet, equivalent in sound to the English v. In some of the earlier Greek dialects the old y was a kind of aspirate, which, from its form, like one capital P over another, was called digamma, and written F. The Pelasgians carried this aspirate into Italy, where it remained in Latin as a real consonant, in such words as vinum, ovum, from the Greek Foryoc, Fc.loy. The digamma had disappeared as a character from the Greek language before the days of Homer.