If a negative is immersed in extremely dilute hydrofluoric acid contained in an ebonite dish, say half a teaspoonful to half a pint of water, the film very soon becomes loosened, and floats off the glass, this circumstance being due to the solvent action which the acid exercises upon the surface of the plate as soon as it has penetrated the film. If the floating film be now caught upon a plate which has been slightly waxed, and it is allowed to dry on this plate, it will become quite flat and free from wrinkles. To wax the plate, it should be held before the fire until it is moderately hot, after which it is rubbed over with a lump of wax, and the excess is polished off with a piece of flannel. When the film is dry, it will leave the waxed glass immediately, if one corner is lifted by means of a penknife. The film will become somewhat enlarged during the above-described operation; but, by taking suitable precautions, this enlargement may be avoided. It is also convenient to prepare the hydrofluoric acid extemporaneously by the action of sulphuric acid on fluoride of sodium; and, in many cases, it is advisable to thicken up the film by an additional layer of gelatine. The following directions embody these points. The negative, which must be unvarnished, is leveled, and covered with a layer of warm gelatine solution (one in eight) about as thick as a sixpence. This done, and the gelatine set, the plate is immersed in alcohol for a few minutes in order to remove the greater part of the water from the gelatinous stratum. The next step is to allow the plate to remain for five or six minutes in a cold mixture of one part of sulphuric acid with twelve parts of water, and in the mean time two parts of sodium fluoride are dissolved in one hundred parts of water, an ebonite tray being used. A volume of the dilute sulphuric acid equal to about one-fourth of the fluoride solution is next added from the first dish, and the plate is then transferred to the second dish, when the film soon becomes liberated. When this is the case, it is placed once more in the dilute sulphuric acid. After a few seconds it is rinsed in water, and laid on a sheet of waxed glass, complete contact being established by means of a squeegee, and the edges are clamped down by means of strips of wood held in position by American clips or string. All excess of sulphuric acid may now be removed by soaking the plate in methylated alcohol, after which it is dried. It is as well to add a few drops of ammonia to the last quantity of alcohol used. The plate bearing the film negative is now placed in a warm locality, under which circumstances a few hours will suffice for the complete drying of the pellicular negative, after which it may be detached with the greatest ease by lifting the edges with the point of a penknife.--_Photo. News_. * * * * * NEW ANALOGY BETWEEN SOLIDS, LIQUIDS, AND GASES, By W. SPRING.