* "I call this substance bromide of lime, although there is a difficulty as to the composition of bleaching powder, and which would also apply to the compounds I describe. Some chemists regard the chloride of lime to be a compound of lime, water, and chlorine. Balard thinks it is a mixture of hypochlorite of lime and chloride of calcium; and the view of Millon and Prof. Graham is, that it is a peroxide of lime, in which one equivalent of oxygen is replaced by one of chlorine".
" The great advantage of this compound is, that it may be used continuously for a fortnight without renewal; and, unlike bromine water, its action is unaffected by the ordinary changes of temperature".
The advantages of a dry material are so great, that the bromide of lime is now commonly used.
By the employment of these agents a sensitive coating is produced, upon which actinic changes are almost instantaneously made. The modes of proceeding to prepare the plates are similar to those already named.
The time necessary for the plate to be exposed to the action of the bromine water, if it be used, must be determined by experiment, for it will vary according to the size of the box and the quantity of liquid used. It is ordinarily between thirty and sixty seconds, the time varying with the temperature of the atmosphere: when once determined, it will be constant with the same box, the same strength of solution, and the same temperature.
The method of coating the plate with bromine from the water which is most approved is as follows:—Place a pan in a properly prepared box, fill a pipette with bromine water, and pour it carefully from this into the pan, then close the vessel with a glass plate : the liquid must cover evenly the bottom of the pan; if not level, it must be adjusted: the level will be easily seen through the glass slide. When everything is thus arranged, the plate, previously iodized, is to be placed in its frame over the pan, the slide withdrawn, and the necessary time counted; after this has elapsed, the slide should be shut, and the plate immediately placed in the dark box of the camera.
* " It is better to count time both over the iodine and the bromide of lime : the exposure of the plate to the iodine, after it has received its proportion of bromine, should be one-third of the time it took to give it the first coating of iodine. We have found that if less iodine than this be allowed to the plate it will not take up so much mercury, neither will the picture produced be so bold and distinct".
For a second operation, this bromine water must be thrown away, and a fresh quantity used. The bottle containing the bromine water should be kept away from the direct light of the sun, and care should be taken that no organic matter fall into the bottle, such as grease, chips of cork, etc. These enter into new combinations with the bromine, and lead to error as to the amount in solution.
Daguerre himself introduced some very considerable improvements in the process of iodizing. He avoided the use of metal strips, and gave some curious experiments on the action of edges, grooves, etc., in determining the deposition of vapour. M. Daguerre stated that, but for the difficulty of fixing them, the bands might be very much reduced in size; for it is sufficient for them to produce their effect that there be a solution of continuity between them; and this is proved by the fact that nearly the same result is obtained by engraving at the 1/8th of an inch from the edge of the plate a line deep enough to reach the copper. The objections to this are, that during the polishing process the engraved line is filled with dust, and it retains water, which sometimes occasions stains. He then proposed, as a very great sunplification of this process, that the plate should be laid flat in a shallow box containing two grooves, one to receive the plate, and the other a board saturated with iodine. Around the plate he places a border of either powdered starch or lime, and the iodine descends from the board to the tablet. The starch or lime absorbs the iodine with avidity, and thus prevents its attacking the edges of the silver, and the vapour is diffused with perfect evenness over it. Another advantage is, that the saturated board may be used for several days in succession, without being renovated.
M. Seguier somewhat modified even this process. A box of hardwood, varnished internally with gum lac, contains a lump of soft wood, furnished with a card of cotton sprinkled with iodine. Upon this is placed a plate covered with card-board on each of its faces. One of these card-boards furnishes, by radiation, to the metal the vapour of iodine, while the other returns to the cotton that which it had lost. It suffices to turn the plate from time to time, in order that the operation may go on with equal rapidity. A plate of glass is placed upon the upper card-board, where it is not operated on. The plate is sustained a little above the charged cotton by frames of hardwood varnished with gum lac. By increasing the distance between the cotton and the plate, or the contrary, we are enabled to suit the arrangement to the temperature of the season, and thus always operate with facility and promptitude. M. Seguier also states, that a single scouring with tripoli, moistened with acidulated water, is sufficient to cleanse the plates thoroughly, thus doing away with the tedious process of scouring with oil, and afterwards the operation of heating the tablet over a spirit lamp. M. Soliel has proposed the use of the chloride of silver to determine the time required to produce a good impression on the iodized plate in the camera. His method is to fix at the bottom of a tube, blackened within, a piece of card, on which chloride of silver, mixed with gum or dextrine, is spread. The tube thus disposed is turned towards the object of which we wish to take the image, and the time that the chloride of silver takes to become of a grayish slate colour, will be the time required for the radiations in the camera to produce a good effect on the iodated silver.
These remarks have been introduced as supplementary to the generally approved modes, as they are suggestive in themselves of still further improvements.