Caoutchouc dissolved in mineral Naphtha to the consistence of thick glue or gold-beaters' size, is employed by many for mounting Photographic Prints; it may be obtained at the varnish shops, and is sold in tin boxes. The mode of using it is as follows:--with a broad brush made of stiff bristles, apply the cement to the back of the picture; then take a strip of glass with a straight edge, and by drawing it across the paper, scrape off as much as possible of the excess. The print will then be found to adhere very readily to the cardboard, without causing expansion or cockling; and any portion of the cement which oozes out during the pressing may, when dry, be removed with a penknife without leaving a stain. REMARKS UPON THE WANT OF CORRESPONDENCE BETWEEN THE FORMULA OF DIFFERENT OPERATORS. The formulæ for Positive printing given in the works on practical Photography exhibit great variety; and it has been proposed to attempt to reduce them to more uniform proportions. This cannot however easily be done, both on account of the difference in the structure and preparation of the various Photographic papers, and also because the mode of applying the solutions is not always the same. Take as an illustration the following process, which has long been recommended for its simplicity, and which is in every respect a good one:--Dissolve 40 grains of Chloride of Ammonium in 20 ounces of Distilled Water, and _immerse_ about a dozen sheets of Towgood's Positive paper, removing air-bubbles with a camels'-hair brush. When the last sheet has been placed in the liquid, turn the batch over and take them out one by one, so that each sheet, remaining in the liquid at least ten minutes, may be thoroughly saturated. When dry, excite by brushing with a 40 or 60-grain solution of Ammonio-Nitrate of Silver in the usual way. Now this formula contains less than one-fifth of the amount of salt often employed, and if a thick foreign paper sized with starch, such as Canson's Positive, were _floated_ upon such a salting Bath, it would be difficult to obtain a good picture. By _immersing_ however a paper sized with Gelatine like the one recommended, a much larger quantity of salt is retained upon the surface, and the film is sufficiently sensitive. There are three modes of applying solutions, viz. by brushing, floating, and immersion. The quantity of solution left on the paper varies with each, and consequently each requires a different formula. Immersion in a strong salting Bath tends to give a coarse picture wanting in definition; whereas the plan of brushing a weak salting solution, produces a paper deficient in sensitiveness, and yielding a pale red image without proper depth of shadow.