Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/865

PHOTOGRAPHY 829 Maddox had tried emulsifying the silver salt in gelatin, and had produced negatives of rare excellence, as the present writer can testify from personal knowledge. In November 1873 Mr King described a similar process, getting rid of the soluble salts by washing. Efforts had also been made in this direction by Mr Burgess in July 1873. Mr R. Kennett in 1874 may be said to have been the first to put forward the gelatin emulsion process in a practical and workable form, as he then published a formula which gave good and quick results. It was not till 1878, however, that the great capabilities of silver bromide when held in suspension by gelatin were fairly known ; in March of that year Mr C. Bennett showed that by keeping the gelatin solution liquid at a low temperature for as long as seven days extraordinary rapidity was conferred on the sensitive salt. The molecular condition of the silver bromide seemed to be altered, and to be amenable to a far more powerful developer than had hitherto been dreamt of. In 1874 the Belgian chemist Stas had shown that various modifications of silver bromide and chloride were possible, and it seemed that the green molecular condition (one of those noted by Stas) of the bromide was attained by prolonged warming. It may in truth be said that the starting-point of rapid plates was 1878, and that the full credit of this discovery should be allotted to Mr C. Bennett. Both Kennett and Bennett got rid of the soluble salts from the emulsion by washing ; and in order to attain success it was requisite that the bromide should be in excess of that necessary to combine with the silver nitrate used to form the emulsion. In June 1879 Abney showed that a good emulsion might be formed by precipitating a silver bromide by dropping a solution of a soluble bromide into a dilute solution of silver nitrate. The supernatant liquid was decanted, and after two or three washings with water the precipitate was mixed with the proper amount of gelatin. Dr van Monckhoven of Ghent, in experimenting with this process, hit upon the plan of obtaining the emulsion by splitting up silver car bonate with hydrobromic acid, leaving no soluble salts to be extracted. He further, in August 1879, announced that he had obtained great rapidity by adding to the bromide emulsion a certain quantity of ammonia. This addition rapidly altered the bromide of silver from its ordinary state to the green molecular condition referred to above. At this point we have the branching off of the gelatin emulsion process into two great divisions, viz., that in which rapidity was gained by long-continued heating, and the other in which it was gained by the use of ammonia a subdivision which is maintained to the present day. Photographers opinions as to the respective merits of the two methods are much divided, some maintaining that the quality of the heated emulsion is better than that produced by alkalinity, and vice versa. We may mention that in 1881 Dr Herschell intro duced a plan for making an alcoholic gelatin emulsion with the idea of inducing rapid drying of the plates, and in the same year Dr H. Vogel of Berlin brought forward his ideas for combining gelatin and pyroxylin together by means of a solvent which acted on the gelatin and allowed the addition of alcohol in order to dissolve the pyroxylin. This method was called " collodio-gelatin emulsion," and apparently was only a shortlived process, which is not sur prising, since its preparation involved the inhalation of the fumes of acetic acid. The warming process introduced by Bennett was soon superseded. Colonel Stuart Wortley in 1879 announced that, by raising the temperature of the vessel in which the emulsion was stewed to 150 Fahr., instead of days being required to give the desired sensibility only a few hours were necessary. A further advance was made by boiling the emulsion, first practised, we believe, by Mr Mansfield in 1879. Another improvement was effected by Mr W. B. Bolton by emulsifying the silver salt in a small quantity of gelatin and then raising the emulsion to boiling point, boiling it for from half an hour to an hour, when extreme rapidity was attained. It would be impossible to enumer ate many minor improvements in this process that have from time to time been made ; it is sufficient to have stated in historical sequence the different important stages through which it has passed. It may be useful to give an idea of the relative rapidities of the various processes we have described. Daguerreotype, originally half an hour s exposure. Calotype 2 or 3 minutes , , Collodion 10 seconds ,, Collodion emulsion 15 seconds ,, Rapid gelatin emulsion 1*5 th second , , By this it will be seen what advances have been made in the art of photography during the forty-five years of its existence. The following is an outline of two representative processes. All Gelatin operations should be conducted in light which can act but very emul- slightly on the sensitive salts employed, and this is more necessary sions. with this process than with others on account of the extreme ease with which the equilibrium of the molecules is upset in giving rise to the molecule which is developable. The light to work with, and which is safe, is gaslight or candlelight passing through a sheet of Chance s stained red glass backed by orange paper. Stained red glass allows but few chemically effective rays to pass through it, whilst the orange paper diffuses the light. If daylight be em ployed, it is as well to have a double thickness of orange paper. The following should be weighed out : , J Autotype or other hard gelatin 100 ( Nelson s No. 1 gelatin 100 Nos. 3 and 5 are rapidly covered with water or washed for a few seconds under the tap to get rid of any adherent dust. No. 2 is dissolved in 1^ oz. of water, and a little tincture of iodine added till it assumes a light sherry colour. No. 1 is dissolved in 60 minims of water. No. 4 is dissolved in oz. of water, and No. 3 is allowed to swell up in 1 oz. of water, and is then dissolved by heat. All the flasks containing these solutions are placed in water at 150 Fahr. and carried into the "dark room," as the orange- lighted chamber is ordinarily called ; Nos. 3 and 4 are then mixed together in a jar or flask, and No. 2 added drop by drop till half its bulk is gone, when No. 1 is added to the remainder, and the double solution is dropped in as before. When all is added there ought to be formed an emulsion which is very ruddy when examined by gaslight, or orange by daylight. The flask containing the emul sion is next placed in boiling water, which is kept in a state of ebullition for about three-quarters of an hour. It is then ready, when the contents of the flask have cooled down to about 100 Fahr. , for the addition of No. 5, which should in the interval be placed in 2 oz. of water to swell and finally be dissolved. The gelatin emulsion thus formed is placed in a cool place to set, after which it is turned into a piece of coarse canvas or mosquito -netting made into a bag. By squeezing, threads of gelatin containing the sensitive salt can be made to fall into cold water ; by this means the soluble salts are extracted. This is readily done in two or three hours by frequently changing the water, or by allowing running water to flow over the emulsion -threads. The gelatin is next drained by straining canvas over a jar and turning out the threads on to it, after which it is placed in a flask, and warmed till it dis solves, half an ounce of alcohol being added. Finally, it is filtered through chamois leather or swansdown calico. In this state it is ready for the plates. The other method of forming the emulsion is with ammonia. The same quantities as before are weighed out, but the solutions of Nos. 2 and 3 are first mixed together and No. 4 is dissolved in 1 oz. of water, and strong ammonia of specific gravity 880 added to it till the oxide first precipitated is just redissolved. This ammoniacal solution is then dropped into Nos. 2 and 3 as previously described, and finally No. 1 is added. In this case no boiling is required ; but to secure rapidity it is as well that the emulsion should be kept an hour at a temperature of about 90 Fahr., after which half the total quantity of No. 5 is added. When set the emulsion is washed, drained, and redissolved as before ; but in order to give tenacity to the gelatin the remainder of No. 5 is added before the addition of the alcohol, and before filtering. Coating the Plates. Glass plates are best cleaned with nitric Coating acid, rinsed, and then treated with potash solution, rinsed again, the plate.