Preparation Of The Emulsion. In the dark-room, lighted by ruby-colored glass, melt separately, over a water-bath, using as mild a collodion as rapid as gelatin. I have not the time to make the experiments myself which would be necessary. Meta-gelatin is probably the chief mechanical cause of the increase of sensitiveness in gelatin emulsion. Being intimately mixed with the gelatin, it renders the film more pervious to the developer by being washed out, and yet perhaps not carrying away with it the particles of silver bromide acted upon by the light. There is an interesting field for experiment in this direction. If the particles of silver bromide are washed out as well as the meta-gelatin, this may explain the want of density found in some highly sensitive films. Third. Yet, again, has temperature an important influence in gelatin work. A third canon is this: The putrefactive and fermentative conditions of gelatin in the moist state are induced directly as the temperature at which the moist gelatin is kept and placed, or manipulated, in circumstances favorable to the admission of germs from the atmosphere. Dr. Maddox - who has - distinguished himself as a pioneer in gelatino-bromide of silver emulsion work - has been even more distinguished in microphotography. I would suggest to him a fertile field in the examination and photographic delineation of gelatin emulsion which has been kept, and to which the atmosphere has had access.
An emulsion free from living organisms - bacteria, vibrios, etc. - will be found a sine qua non for good work. I am inclined to think, if the microscope were used more freely in the investigation of emulsions, we should find that some of the puzzling difficulties of gelatino-bromide of silver might be traced to gelatin in process of putrefaction or fermentation, in consequence of having been kept without the addition of an antiseptic. As last words in regard to this subject of temperature, I would say, gelatin boiled is gelatin spoiled. - Duncan C. Dallas.
328. Dr. J. Nicol gives a description of a very convenient arrangement, devised by Mr. J. G. Tunny, of Edinburgh, for keeping the various vessels containing emulsions at the proper temperature. This he has called a " hot-water cabinet." It will be seen from the accompanying sketch, Fig. 76, that the cabinet consists of a cylinder of japanned tin plate, nine inches deep and thirteen inches in diameter. In the top are six circular holes of various sizes, suitable for the vessels intended to be kept warm, including one funnel, one or more measures, several beakers, and the emulsion bottle. In each of the circular holes is inserted and firmly soldered a tin cylinder, closed at the bottom, and of such a length as just to hold the article for which it is intended. The mouth of each of the cylinders projects sufficiently above the top of the cylindrical body of the cabinet to form a shoulder on which to slip a cap or cover, whereby light may be excluded from all or any particular one at will. Immediately in front of the funnel - holder - which is, of course, not a cylinder heat as possible (that of a lamp whose light is hid is the best), the following two preparation:
A. - Water,......
. 120 parts.
Bromide of Ammonium...................
Bromide of Zinc,..................
B. - Water,..................
Nitrate of Silver..................
These two solutions bring made, pour, drop by drop, constantly agitating, the second solution into the first Allow the mixture to remain but a cone - a portion of the side of the cabinet is cut away, and covered by a sliding shutter of tin. This is intended to receive the bottle or vessel into which the emulsion is being filtered, and a half cylinder passing behind the bottle and reaching up to the funnel-holder keeps the whole water - tight. There is also a small tube, in which may be inserted a thermometer for the proper regulation of the temperature. For all or most of the operations connected with the preparation of gelatin plates I cannot imagine, and certainly have never seen, anything so exceedingly convenient. By means of a small gas- or lamp-flame, the temperature of the water may be maintained at any required degree, and if covered with a jacket of thick felt or other suitable non - conducting substance, an almost uniform temperature may be kept up for many hours without any flame at all.
In boiling the gelatin emulsion, it is of great importance that the vessel containing it should be well covered with water to enable the emulsion itself to boil thoroughly, and in order to effect this some arrangement is required to prevent the bottle or flask from turning upside down. To get over this difficulty a heavy glass bottle is often used, much to the discomfort of the worker, who generally breaks four such bottles out of every five. To those who require a simple plan of steadying a flask, I recommend the accompanying sketch of an arrangement which is easily made and answers perfectly. It consists simply of a collar formed from a piece of stout sheet-lead bent to the proper shape of, and made half an inch deeper than, the glass flask, and also pierced with several holes to allow the water to flow easily through it. When placed in the boiling water the "collar " prevents the flask from turning over, which, if not filled too full, is also hindered by its buoyancy from touching the bottom of the outer vessel. Setting aside the question of breakage, I may observe that an ordinary chemical flask should be always used in preference to a bottle, as by its use the emulsion is heated far more quickly, and can, without risk, be immediately cooled under the tap when the boiling is completed. Thus a great saving of time, and sometimes patience, is effected. - H. M anfield.
Thinking that something more mechanical than the rougher method, mostly recommended, of adding silver to the bromized gelatin and then shaking your arms off, was requir. I set about making a simple piece of apparatus; and, as it answers the purpose admirably. I give a sketch of it, the sizes of which can be altered to suit requirements. The frame, a, is made of wood, the base, B, of something heavier to keep it steady when in use. Wood would do if loaded with lead underneath. The large and small band-pulleys are likewise of wood; the stirring-rod, C, is made, as well as the toothed piece on the bottom. This over a very mild water-bath for three days. Now pour the solution into a glass dish, which is to he placed in as cold a place as possible; in summer, ice is necessary. When the bromized gelatin has become very firm, stirrer, used in conjunction with an injector, gives a beautifully fine-grained, creamy emulsion, and by using an India-rubber ball on the tube, F, you can force the silver solution into the gelatin in the form of a fine spray, or drop by drop, as you think fit. A word on frilling, one of the chief causes of which I believe to be dirty glasses - that is to say chemically dirty - by allowing too long a time to elapse between the cleaning and coating of the plates. I quite agree with a plan, no doubt, adopted by many - that is, to rummage up all old plates and have one good wash up, after which they should be packed up and stored away for future use. But it is a great mistake to coat plates even twenty-four hours after such washing without a preliminary polish up. The plan I adopt is this: while the gelatin emulsion is soaking I look out my dozen or two plates, as the case may be, dust on a little fine tripoli powder on the side to be coated, and polish it off with a cloth I keep for the purpose, and which is pretty well charged with the powder. It will be found that this gives a tooth to the plate that gelatin emulsion will stick to with great tenacity. A quarter of an hour is quite sufficient to go over two dozen plates. This precaution taken, coupled with the use of chrome-alum in the emulsion, I will warrant to prevent frilling. - S. Rogers.