Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/864

 828 bromide or silver nitrate, were removed, when the film was exposed wet, or allowed to dry and then exposed. The rapidity of these plates was not in any way remark able, but the process had the great advantage of doing away with the sensitizing nitrate of silver bath, and thus avoiding a tiresome operation. The plates were developed by the alkaline method, and gave images which, if not primarily dense enough, could be intensified by the ap plication of pyrogallic acid and silver nitrate as in the wet collodion process. Such was the crude germ of a method which was destined to effect a complete change in the aspect of photographic negative taking ; l but for some time it lay dormant. In fact there was at first much MoJifi- to discourage trial of it, since the plates often became cations veiled on development. Mr Carey Lea of Philadelphia, and Mr W. Cooper, jun., of Reading, may be said to have given the real impetus to the method. Mr Carey Lea, by introducing an acid into the emulsion, established a practicable collodion emulsion process, which was rapid and at the same time gave negative pictures free from veil. To secure the rapidity Carey Lea employed a fair excess of silver nitrate, and Colonel Wortley gained further rapidity by a still greater increase of it ; the free use of acid was the only means by which this could be effected without hopelessly spoiling the emulsion. It may be well to mention that the effect of the addition of the mineral acids such as Carey Lea employed is to prevent the forma tion of (or to destroy when formed) any sub-bromide or oxide of silver, either of which acts as a nucleus on which development can take place. Captain Abney first showed the theoretical effect of acids on the sub-bromide, as also the effect of oxidizing agents on both the above compounds (see below). A more valuable modification was introduced in 1874 by Mr W. B. Bolton, one of the originators of the process, who allowed the ether and the alcohol of the collodion to evaporate, and then washed away all the soluble salts from the gelatinous mass formed of pyro xylin and. sensitive salt. After washing for a considerable time, the pellicle was dried naturally or washed with alcohol, and then the pyroxylin redissolved in ether and alcohol, leaving an emulsion of silver bromide, silver chloride, or silver iodide, or mixtures of all suspended in collodion. In this state the plate could be coated and dried at once for exposure. Sometimes, in fact generally, preservatives were used, as in the case of dry plates with the bath, in order to prevent the atmosphere from rendering the surface of the film spotty or insensitive on development. This modification had the great advantage of allowing a large quantity of sensitive salt to be prepared of precisely the same value as to rapidity of action and quality of film. A great advance in the use of the collodion bromide process was made by Colonel Stuart Wortley, who in June 1873 made known the powerful nature of a strongly alkaline developer as opposed to the weak one which up to that time had usually been employed. The brief exposure necessary for a collodion emulsion plate, or indeed any dry plate, had not been recognized till the introduction of this developer. This at once placed in the hands of photographers an instru ment which by judicious use enabled them to shorten the time of exposure, of their plates and to render possible effects which had before been considered out of the question. As an example of the preparation of a collodion emulsion and the developer usually employed with it we give the following, 2^ oz. of alcohol, 5 oz. of ether, 75 grains of pyroxylin. In 1 oz. of alcohol are dissolved 200 grains of zinc bromide ; 2 it is then acidulated with 4 or 5 drops 1 An account of Mr Sayce s process is to be found in the 1 hoto- rjraphic News of October 1 865, or the Photographic Journal of the same date. 2 The advantages of this salt were pointed out by Mr. Warnerke in 1875. 2. 3. of nitric acid, and added to half the above collodion. In 2 drachms of water are dissolved 330 grains of silver nitrate, 1 oz. of alcohol being added. The silvered alcohol is next poured into the other half of the collodion and the brominized collodion dropped in, care being taken to shake between the operations. An emulsion of bromide of silver is formed in suspension ; and it is in every case left for 10 to 20 hours to what is technically called &quot;ripen,&quot; or, in other words, to become creamy when poured out upon a glass plate. When the emulsion has ripened it may be used at once or be poured out into a fiat dish and the solvents allowed to evaporate till the pyroxylin becomes gelatinous. In this state it is washed in water till all the soluble salts are carried away. After this it may be either spread out on a cloth and dried or treated with two or three doses of alcohol, and then redissolved in equal parts of alcohol (specific gravity, 805) and ether (specific gravity, 720). In this condition it is a washed emulsion, and a glass plate can be coated with it and the film dried, or it may be washed and a preservative applied. An ex cellent preservative introduced by Colonel Stuart Wortley is as follows : 1. Salycin, a saturated solution in water. / Tannin 60 grs. 1 Distilled water 1 ox. / Gallic acid 48 grs. Water 1 ox. To make the preservative, take 2 oz. of No. 1, 1 oz. of No. 2, i oz. of No. 3, 40 grains of sugar, and 7 oz. of water. The plates are immersed in this solution and dried. It is often necessary to give the plate a previous coating with very dilute albumen or gelatin in order to make the film of collodion adhere during development, which can be effected by the strong alkaline developer, or by the ferrous oxalate developer, previously noticed. The type of a useful alkaline developer is as follows : , f Pyrogallic acid 96 grs. Alcohol la/. / Potassium bromide 12 grs. &quot;i Water distilled 1 ox. / Ammonium carbonate 80 grs. Water 1 oz. To develop the plate 6 minims of No. 1, -|- drachm of No. 2, and 3 drachms of No. 3 are mixed together and made to flow over the plate after washing the preservative off under the tap. Sometimes the development is conducted in a flat dish, sometimes the solution is poured on the plate. :! The unreduced salts are eliminated by either cyanide of potassium or sodium hyposulphite. Intensity may be given to the image, if requisite, either before or after the &quot; fixing &quot; operation. Where resort is had to ferrous oxalate development, the developer is made in one of two ways (1) by saturating a saturated solution of neutral potassium oxalate with ferrous oxalate, and adding an equal volume of a solution (10 grains to 1 oz. of water) of potassium bromide to restrain the action, or (2) by mixing, according to Eder s plan, 3 volumes by measure of a saturated solution of the potassium oxalate with 1 volume by measure of a saturated solution of ferrous sul phate, and adding to the ferrous oxalate solution thus obtained an equal bulk of the above solution of potassium bromide. The development is conducted in precisely the same manner as indicated above, and the image is fixed by one of the same agents. Gelatin Emulsion Process. The facility with which Gelatin collodion emulsion plates could be prepared had turned all enmlsid investigation into this channel, and collodion was not the pro only vehicle that was tried for holding the sensitive salts in suspension. As early as September 1871 Dr R. L.
 * t For further details the reader is referred to Instruction in Photo

graphy, p. 99. 2. 3.