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722 THE PHOTOGRAPHIC NEWS. LNOvEMIBER 16, 1883. Coaling with Substratum.—Having selected the paper, by preference using thin Saxe which has been kept flat under pressure-boards in a moist atmosphere a short time, coat it as evenly as possible with one of the foregoing mixtures. An easy method of proceeding is to lay the paper, coarse grained side downwards, on a level bench or glass plate, brush the surface to remove dust, then rub one of the sub strata (excepting the first, which is better applied by floating at a temperature of 85° F.) evenly all over the surface by means of a sponge or strong paste-brush, taking care not to leave any of the substance sticking up in lumps. When an even coating has been obtained, suspend in a current of air to dry. If this operation is performed in the evening, the paper will be dry by the next morning if hung up in a moderately dry room. When the coating of starch or other substrata has become dry, it is ready] for the next operation, i.e., coating with the sensitive emulsion. There are several ways of accomplishing this. Mr. Whaite* prefers working the cold emulsion over the paper, in the same manner a paper-hanger would his paste, afterwards drawing the sheet or length slowly, but steadily, over an oven containing boiling water. For papers not more than twelve inches wide, the ordinary cylindrical hot water foot-warmer answers very well. The water should be as near boiling as possible, and the paper drawn evenly over it (by no means an easy operation in semi-darkness) , or wavy lines, streaks, and unevenness are very likely to ensue. Another plan is one that has been suggested for making small quantities of carbon tissue—i. e., to roll up the paper, and gradually unroll it over a dish containing the gela tine mixture with one hand ; with the other, raising the coated end quickly, in the form of a loop ; great care is necessary to prevent the edges becoming immersed, or the surface touching the dish after leaving the emulsion. It is also necessary to keep a steady onward movement, the slightest stoppage producing a thickening, which will after wards run in tear drops. By a simple mechanical con trivance of passing the paper over a wooden roller, having an oscillating as well as a rotating movement, this defect could be easily overcome, the plan having much to recom mend it to the notice of amateurs and producers on a small scale. In our next we shall describe coating more fully by another method. THE NATURE OF GREEN FOG. We recently, in discussing this subject, informed our readers what we believed to be the actual nature of this bug-bear, promising to give further particulars in con firmation of our view. The rough experiment that we recommended to be performed in a test-tube we now extend in other directions. That experiment, as will be remembered, was to add pyro to a solution of chloride or bromide of silver in ammonia. We ask those interested in this subject to prepare a small quantity of bromide of silver, using excess of bromide. Wash the precipitate well, and add ammonia in sufficent quantity to nearly, but not quite, dissolve all the silver salt. After well agitating the vessel, allow the undissolved silver bromide to settle at the bottom, and pour off the clear solution into a well-stoppered bottle. Now take a clean working gelatino-bromide plate, and over one portion of it pour a small quantity of this solution diluted with five or six times its volume of water. Stand the plate up to drain, allow it to dry, and then develop either with or without exposing it to light. On the part that has been treated with the solution, green fog will appear, being most noticeable, we need scarcely say, in the shadows, the part least acted upon by light. Next, dissolve twenty grains of gelatine in an ounce of water, and add a drop or two of the solution. Coat a • See Yxan-Boox os Puoroonariry, 1883, few plates and allow them to set. Take one of the plates when thoroughly set, and apply the developer for a minute or so. On taking the plate out of the dish and examining it in front of a white light, it will be found to be green by reflected, and pink by transmitted light, showing, in fact, the two characteristic features of green fog. If a larger quantity of the solution be added to the dis solved gelatine, green fog may be proportionately increased, until at last, by getting a sufficient amount of it in the gela tine, we may arrive at a stage when the characteristic colour of the fog disappears, and we get an ugly brown colour by reflected, and purplish colour by transmitted light, con sequent on the greater coarseness of the particles of pre cipitated silver. Our most successful result with a plate prepared as above was obtained by applying a weak solution of pyro in alcohol, instead of the ordinary developer. But the prettiest experiment we give to our readers is the follow ing :—Take twenty grains of gelatine, dissolve it in two ounces of water, and pour it into a four-ounce bottle, adding to it about half a drachm of the solution of bro mide of silver in ammonia. In another two ounces of water, dissolve half a grain of pyro, and pour it slowly with shaking into the bottle containing the gelatine and green fog solution. The operation requires care as in emulsion making, but if carefully carried out, an emulsion of veritable green fog will be the result. This may be examined at leisure in any light, and in any mannet that the reader thinks suitable ; a very good way is to pour some of it out upon a glass plate. Of course, a stronger emulsion than this may be made, but the more solution that is put in, the more care is required to keep the precipitate in the fine state of division without which we could hardly call it green fog. A yet more successful re sult may be obtained by using a solution of chloride of silver in ammonia, bat on account of the greater solubility of the chloride a much smaller quantity must be used. In these experiments we have succeeded in obtaining silver in a fine state of division, in which state it resembles the green fog produced on developing certain plates. Our readers well know that the longer a plate is left in the developer, and the larger the amount of ammonia in that developer, the more likelihood is there of green fog appear ing. Are we assuming too much in believing that where green fog appears on prolonged development it is caused by the precipitation of silver from the minute quantity of silver bromide which the ammonia has had time to dissolve out of the film ? SULPHUR AND SOME OF ITS REACTIONS. The general view which regards sulphur as being a prime mover in the fading of prints on albumenized paper is probably a correct one; and the circumstance that pic tures produced in an albumen film are especially liable to fade is confirmatory of the usual opinion. Although that sulphur which forms an integral constituent of albumen is held in combination with carben, hydrogen, nitrogen, and oxygen, there is reason to believe that it exists in a partially unoxidised state, and not in the harmless condi tion of a sulphate. The observations of most who have studied the condi tions under which prints made upon albumenized paper fade, tend to prove that as long as the prints are care fully guarded against moisture, there is but little risk of rapid deterioration. As bearing upon this subject, some recent observations of Cross and Higgin possess considerable interest, as these investigators have proved that water exercises a notable action upon sulphur. It must be understood that the action of water upon sulphur existing in combi nation, as in albumen, is not necessarily identical with the action of water upon free sulphur ; but there is likely to ne a correspondence up to a certain point. The investigator