We hare already frequently mentioned the gelstino-bromide plates, now I wish to syy few words about their prepare The modern plates are prepared by cov ing gins with a film of gelatine of bre ide or chi Ths bromide emulsion is chiefly although for certain purposes the gelatin chlorite emulsion is. preferred pare this emulsion so is aided to " potassium promi containing some gelatine. gelatine is the cause of emulsification; the silver bromide precipitated from a aolution of potassium bromide in plain water, which is added silver nitrate, becomes granular and oon subsides, while that Precipitated in the gelatine solution emall sities.
the industrial preparation of the me iodide wf potassium is also the potassium bromide solution, and to the silver nitrate solution: is "dd generally a small quantity of ammonia The emulsion is then ubjected to special operations, which are called "ripening the emulsion" which chiefly consists of yontly heating the mixture for a certain time. Afterwards the emulsion is washed, specially to eliminate the nitrate of sium produced in the reaction of the nitrate of silver upon the bromide of potassium. Then it is filtered and by means of speci muichines is spread in s regular film over a lass plate or other support. Finally the glass plate is dried, taking the utmost care toward depositing of dust on the plate, and in order to do thie the walls are kept well clean and the floor wet. Moreover the air is drawn from the exterior and filtered. is hardly necessary to add that all the operations described are per-this system of lanterns the luboraare better illuminated than with ordinary lamps with coloured glasses and also the light is better filtered.
THE LATENT OR INVISIBLE IMAGE, Now we my fut question to ourselves, What is the nature of the image which exists on the surface of a sensitive film of 8 gelatine-bromide emulsion after exposure in the camera The difficulties of investigating this subject are evidently great, owing to the extremely minute amount of silver bromide which undergoes photo-decomposition on the surface of what is in itself nothing more than a film, The chloride, "the bro and the of silver if ex to light for a sufficient length of time become darkened, but the bromide of silver of the glass plat, normally exposed to light in order to have the image of an object, is not apparently changed, but the picture becomes visible when submitted to a subsequent operations of development. Moreover, we may give an exposure in the dark camera to a plate a few hundred times more than is necessary without seeing any visible change on ihe #irface of the plate.
Then is it " physical action or a chemical one? I regret to say that is not possible to answer in the present stage of our knowledge. The prohi-m has attracted attention of wany studious ant scientific men since the discovery of the daguerreotype, but it in not yet solved, although now there ix x grest deal of very deep study on the question. Perhaps we are near its solution. It seems probable that the change on the sensitive film which constitutes the latent or invisible image is unt due to the production of a reduction compound of silver, Lut to what is called a "polymerisation" of the molecula of the silver bromide, That is, we may have new compoual which differs only in the number of the atoms in the molecule interesting question.
only mention one of the latest experiments made on this subject by Messrs. Lumidce concerning the action of light' on ye'atine bromide plates at 190° C. under 0°; this lowest temperature being obtained with liquid air, At this temperature the plate requires an expes tire say of 50 to 40+ times longer than at ordinary temperature. This lick of sensitiveness at a very low temperature nay be perfectly understood in the hypothesis of the polymerisation, although it does not contradict the hypo thesis of x chemical action,
Text read from the printed page and cleaned; old type leaves the occasional error.