Various compounds, called accelerating liquors, have been introduced, in all of which we have combinations in various proportions of either bromine and iodine, or chlorine and iodine, and sometimes of the three. These are known by the names of Eau Bromeé, or Bromine Water, Bromide of Iodine, Redman's Sensitive Solution, Hungarian Liquid, and Woolcott's Accelerating American Fluid. These accelerating compounds are employed after the plate has been subjected to the vapour of iodine. They all require to be diluted with water until about the colour of pale sherry. The plate is exposed to the influence of the vapour which is escaping from the solution used in the same manner as with the iodine, but the colour to be attained differs according to the solution employed. An iodizing box is shown at c, Fig. 63 : at the bottom of this some iodine is strewed, and in general it is covered with a little sand or a card;—the object of this is to avoid an irregular action on any part of the plate: the box being adjusted with a cover, the iodine is preserved from evaporation and lasts a long time. When the plate has assumed its fine straw yellow in the iodine box, it is removed to the action of the accelerating agents, liquid or otherwise, as the case may be. The following rules will guide the experimenter in using the different preparations. If bromide of iodine be used as the accelerating agent, the plate should remain over the iodine until it is of a pure yellow tint, and over the bromine till of a deep rose colour. By observing the time of exposure necessary to render a plate sensitive, any number of plates may be prepared exactly alike, provided that the same quantity of the solution, always of an uniform strength, be put into the pan. By using a much weaker solution a longer exposure is then necessary, but the plate becomes more evenly covered, and there is less danger of having it too much or too little acted upon by the accelerator. The same remark will apply to other accelerating solutions. If Redman's solution, or the Hungarian liquid, a pale yellow and light rose will be found most sensitive. As a general rule, if the yellow colour produced by the iodine be pale, the red should be pale also; if deep, the red must incline to violet. When several plates are to be prepared at one time, the same solution will serve for all; but it seldom answers to preserve the mixture for any long time; and its use, after keeping, is one great cause of the failures which so annoy amateurs. The bromine contained in these solutions is very subtile, and escapes, leaving little else but iodine remaining, which will, after some time, give a red colour to the plate, without rendering it sensitive, entirely disappointing the expectations of the operator. Eau bromée, or bromine water, which is very easily prepared, is extensively used on the Continent, and is simple in its use. If a certain quantity of an uniform solution be placed in the pan, for each plate prepared, one observation will suffice to determine the time of exposure; if not, the colour must guide the operator, varying according to the degree of colour obtained over the iodine: thus, if the first colour obtained be a light yellow, the plate should attain a full golden tint over the iodine, and may then be retained over the bromine until it acquires a rose colour. If iodized of a golden yellow, then, in the second operation, it is taken to a pale rose, and in the third to a deep rose. If in the first of a full red, in the second to a deep red, and lastly to a grey; if the first to a deep red, in the second to a light blue, and in the third, to a white, or nearly the absence of all colour.
Experience, however, must invariably guide the operator, as scarcely any two solutions, though professedly the same in character, possess the same properties.
In a pamphlet published by M. Fizeau, bromine-water is recommended to be prepared as follows:—"To prepare a solution of bromine, of a fixed proportion and convenient strength to operate with, I, in the first place, make a saturated solution of bromine in water; this is prepared by putting into a bottle of pure water a great excess of bromine, agitating strongly for some minutes, and before using allowing the bromine to separate. Now, a definite quantity of this saturated water is to be mixed with a definite quantity of plain water, which will give a solution of bromine always of the same strength: this mixture is conveniently made in the following manner:—The apparatus necessary is a dropping tube, which is also required for another part of the process, capable of holding a small definite quantity, and a bottle having a mark to indicate a capacity equal to thirty times that of the dropping tube: fill the bottle with pure water to the mark, then add, by means of the dropping tube, the proper quantity of the saturated solution of bromine.
"The purity of the water is of some importance: the foregoing proportions refer to the pure distilled water, and it is well known that the water of rivers and springs is not pure; but these different varieties can be used as if they were absolutely pure water by adding a few drops of nitric acid till they taste slightly acid; two or three drops to the pint is generally sufficient.
"The liquid produced, which is of a bright yellow colour, ought to be kept in a well-stopped bottle; it is the normal solution, and I shall call it simply bromine water, to distinguish it from the saturated solution.
The box I employ for subjecting the plate to the vapour of the bromine water is constructed in the following manner:—It consists of a box lined with a varnish, which is not acted on by bromine; its height is about four inches; the other dimensions are regulated by the size of the plate, which ought to be at least hahf an inch all round, short of the sides of the box; it is composed of three separate portions—the cover, which is the frame holding the plate, the body of the box, and the bottom, upon which is placed the vessel for the bromine; this moveable bottom is slightly hollowed, so that the bromine vessel may always be placed in exactly the same position".