Volltext Seite (XML)
This being done, the negative was pressed between blotting-paper so as to take off adherent water, and immediately held in a metal dish at some distance above a Bunsen burner, the dish being moved about so that it should get uniformly heated. The effect on the plate was almost immediate. It began to steam, and the exposed parts swelled up into high relief, just as we had experienced many a time and oft when necessity had caused us to dry a similar plate rapidly so that it might be quickly printed from. When the maximum effect was reached, we left the plate in its dish on the ring of a retort stand placed some fifteen inches above the burner, and it was quite dry in about twelve minutes. The relief still remained apparent, but we judged it to have been reduced in height more than one-half. So far, the experiment was successful, and in case of accident we repeated it with two more plates with equal success. But we should mention that with another brand of plate we came altogether to grief, the film contracting and leaving the glass bare all round the edges directly heat was applied. Two of the successful plates were now placed in the hands of a skilful professional electrotyper to be covered with the copper deposit, one by a dynamo machine and the other by battery. We are sorry to say that the result was the reverse of successful in both cases. The copper, in both plates, left bare places in the gelatine, so that the copper shell, instead of being perfect, was full of holes. Whether Mr. Sutton adopts some means of hard ening the gelatine, or in some other way protects it from the destructive action of the copper bath, we do not know; but our experiments show that the process is not devoid of difficulties. In a subsequent article we shall point out some modifications in the manipulations by which we think more successful results may be achieved. THE “ SIR PAUL PINDAR.” Tins cut is from a photograph of the old house standing until lately in Bishopsgate Street, and which has now been re-erected in the South Kensington Museum as a specimen of Old London architecture. It is an interesting memorial of the time when merchants used to live in the city, and not in the suburbs. The disappearance of this old landmark, which dates from the time of James I., was referred to in our issue of the 3rd instant. FURTHER NOTES ON ELECTRO-CHEMICAL REVERSALS WITH THIO-CARBAMIDES. BY COLONEL J. WATERHOUSE S.C., ASSISTANT SURVEYOR- GENERAL OF INDIA. As a postscript to my last paper on this subject, published in the Photographic News, it was stated that an experi ment with a pair of silver plates coated with precipitated silver bromide, of which one had been exposed to light and the other not, when connected with a very sensitive galvanometer and immersed in a plain eikonogen-lithia developer, showed a distinct electrical current, the needle deflecting towards the left; but when a pair of similar plates were immersed in the same way in some of the same developer containing five drops per ounce of Professor Reynolds’ compound salt of thio-carbamide and ammonium bromide, the current was reversed and the needle deflected to the right. It was found that with the ordinary deve loper the exposed plate formed the negative pole of the galvanic couple, and produces a negative photographic image, while with the thio-carbamide developer it be comes the positive pole, and produces a positive photo graphic image. This reversal of the current has since been successfully repeated with the same kind of plates and developer, and also with a developer containing thio- sinamine. Silver plates bromised by immersion in bromine water have also shown the reversal of current clearly, and I have been able to obtain it on ordinary gelatine dry plates, though in this case the currents are exceedingly weak. There is, therefore, practically little doubt of the fact of the reversal taking place. With silver plates it was found that the observations were rendered rather uncertain by reason of the com paratively strong currents caused by polarisation, especially after the plates had been used a few times, being cleaned merely by a good rubbing with emery powder and polish ing, and therefore more or less impure on the surface. They can, however, be rendered pure by heating to a red heat, and quenching in dilute sulphuric acid. It is desirable to always perform this operation before using the plates. Experiment having shown that it was possible to obtain evidence with the galvanometer of electrical action between the exposed and unexposed halves of a gelatine dry plate during development with ferrous oxalate, various methods of rendering gelatine dry plates sufficiently conductive to show the currents formed during development were then tried. Among the substances used were :—1. Gilt plum bago, which did not answer very well. 2. Silver bronze powder, which is generally a very good conductor, but did not answer at all in this case ; the current from a battery would not pass. 3. Reduced silver in very fine powder, as deposited from mirror silvering solutions; this is an admirable conductor for ordinary electrotyping purposes, and also answered well in this case when applied wet, and polished after drying so as to give a bright and fairly solid coating. 4. Silver leaf; this answered well, but not quite so well as gold leaf. The gold leaf seems, perhaps, the best material to use, because the gold conducts the current without exercising any chemical action with the developer, which might of itself cause currents. It is not so readily attacked by the sulphur set free at the negative pole as silver is, though some action probably does take place-. Either gold or silver leaf can quite easily be applied to the plates. The following method is effective.
