interferes with the rapidity of the operation; hence, after having polished the plate, he floats it with pure water, and heats it to a high degree over a spirit-lamp, and afterwards pours off this layer of water in such a manner that its surface, on which the film, or any sediment removed, may float, shall not touch the plate; after this, the plate is not again polished. This film has been stated by some to assist in the formation of the images; but experience has shown, that too much attention cannot be bestowed on its removal. The plate, when cold, should be finished off with cotton dipped in a solution of one part of nitric acid to sixteen of water, then with dry and clean cotton, and lastly rubbed upon a velvet buffer.

(162.) The next operation is to form on the surface of the silver the sensitive film. On the success, or otherwise, of this operation every thing depends. The plate, when viewed by a weak artificial light, should present a perfectly yellow and brilliant surface. Several plans have been suggested, by which the iodine may be made to attack the plate equally; but all of them require, on the part of the operator, the closest care and attention. The simplest method is to place in the bottom of a box pieces of iodine, and cover them with a piece of gauze stretched on a frame which fits the box. The polished plate is placed a short distance above the gauze. The cover of the box being closed, all is allowed to remain in this state for two or three minutes, when, on removing the plate, it will generally be found that the required fine yellow surface is produced.

(163.) Mr. Goddard in 1840, and M. Claudet in 1841, succeeded in quickening the sensibility of these plates, by combining bromine or chlorine with the iodine. This is effected in various ways by different operators, the usual method being to iodize the plate in the first instance, and then to expose it to the vapour of bromine, or its combination with chlorine or iodine. (Various preparations are sold as accelerating agents, all of them being some preparation

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of bromine.) The result is the conversion of the yellow iodide of silver into a rose-coloured surface, which we may regard as a bromo-iodide. When the plate is properly prepared in this manner, an extreme degree of sensibility is the result. The original process of Daguerre occupied from 15 to 20 minutes in the production of a picture; but by the method now employed, a second or two, in the ordi nary conditions of sunshine, prove sufficient for the production of portraits in the camera obscura.

It is not necessary that the plate should be used as soon as it is excited, some of our most practised Daguerreotype artists preferring to use a plate which has been prepared for some hours, to one taken directly from the iodizing operation.

(164.) The prepared plate is now placed in the dark chamber of the camera obscura; and being properly adjusted, it is exposed to the agency which is to impress upon it the required images. The length of time necessary for the production of the best effect, varies with the state of illumination, the kind of image to be copied, the condition of the plate, and other things which can only be arrived at by experience. In general, where the plate is prepared, as directed by Daguerre, with the iodine alone, about five minutes is required in ordinary sunshine, to copy any architectural pile; but a much less time is sufficient where bromine is added.

(165.) The plate being removed from the camera obscura, no image should be visible upon it. A change has taken place; but as the colour of the plate is not altered, that change cannot be seen. If the plate in any part is browned by two long an exposure, the lights of the picture are injured. To develope the hidden picture, the plate is placed in a box, at the bottom of which is a little mercury, so arranged that the heat of a spirit-lamp may be applied until the temperature of the mercury is about 167° Fahr. The lamp is then removed; and the plate, which is watched through a glass in the side of the box, by the light of a

taper, allowed to remain until the picture, in all the beauty of its minute details, is perfectly brought out. It is then removed; and it has to be subjected to the operaration of fixing, or of rendering the plate of metal insensible to any further solar agency.

(166.) In forming a Daguerreotype image, it is not absolutely necessary to warm the mercury. Faraday proved that mercury was volatilised at common temperatures; and Moser has pointed out, that if the Daguerreotype plate, after it is taken from the camera, is placed over a vessel of cold mercury for some hours, the image will be brought out in the same manner as if the mercury had been warmed. Moser has likewise shown, that if the mercurial vapour is raised to a temperature between 250° and 300° Fahr., the Daguerreotype picture becomes fixed, and may be strongly rubbed. "It loses a little of its intensity at first, but not afterwards. The images, however, cannot generally resist rubbing with moist materials, as with pɔlishing substances." It is not necessary to have recourse to the ordinary methods for removing the iodine from these fixed pictures, as it can be rubbed off.

(167.) Various theories have been from time to time put forth, on the action of mercury on the silver after it has received its impression by radiant action. The Rev. W. Fowler Kingsley has stated his views in a recent communication on this subject, from which I quote:

"The image of the object to be delineated is now formed upon the plate, and then the plate is exposed in the dark to the action of the fumes of mercury, which in a short time whiten the parts that the light has acted on. It used to be thought that the image was formed by particles of mercury being deposited upon the surface of the silver. This, however, I am sure is quite a mistake; and though I am unable to explain the whole action that takes place, I have no doubt of the nature of it. The vapour of mercury has a decided affinity for

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oxygen, iodine, bromine, and chlorine; when, therefore, the plate is exposed to it, the vapour deprives the iodide of silver of its iodine, and deposits pure silver. When bromine and chlorine are also present, silver is reduced, and at the same time salts of mercury are formed; and I have no doubt that the different colours of the lights, according to the different proportions of iodine, bromine, &c., are due to these salts of mercury; but as the quantity is very minute, it is a matter of great difficulty to detect them,-calomel and the black oxide I have detected. After the plate has been acted upon by the mercury, it is fixed by first removing the unreduced iodide by a bath of hyposulphite of soda, and then by gilding it slightly with hyposulphite of gold. If the action of the light be too strong, metallic silver is formed at once, and then the mercury unites with it, and forms an amalgam that is not as light in colour as the silver reduced in the first instance. This seems to be one of the points where there is hope of gaining by further experiments; for if some way of reviving the silver on those parts of the plate where the light has not actually reduced the silver, can be discovered, without destroying the revived parts, we should be able to make sure of every plate by merely giving time enough to the exposure to the light. I may mention that the vapour of aldehyde gives promise of effecting this object. As a de-oxidizing or de-iodizing agent, it is most powerful, but the difficulty it to restrain its action."

Mr. Kingsley has contributed so much that is of value in the applications of Photography to the microscope, &c., that all his remarks must have considerable value. I cannot but think however, had he tried a few experiments on the deposition of vapours on bodies acted. upon by either Heat or Light, where chemical change was out of the question, as on polished metal plates or glass, he would have modified his remarks. I am perfectly satisfied that Mr. Kingsley is wrong; the phenomena are far less complicated than is usually imagined; and he, indeed, in

one of his later paragraphs in the same paper, indicates, without knowing it, the true action.

The bromo-iodide, or iodide of silver, when exposed to radiant action, is immediately decomposed into metallic silver, and the iodine or bromine liberated attacks the surface of the plate, beneath the sensitive film. Now, the amount of decomposition,—that is, the quantity of silver which is left on the surface in a finely divided metallic state, bears an exact relation to the amount of radiant force which has been acting along certain lines. The vaporized mercury immediately unites and forms an amalgam with this silver; and thus are formed the high lights and middle tints of a Daguerreotype picture. If the iodine, which is then combined with the under surface of silver, is allowed to attack the mercury, a very poor result will be obtained; hence the advantage of immediately removing the iodide of silver by the fixing agent employed.

(168.) Fixing may be effected in several ways, all of which depend upon the removal of the iodine from the plate. A strong solution of common salt will do this; but if the plate is placed in such a solution, and then touched with a rod of zinc, the removal is much more rapidly and effectually accomplished. By far the best plan, however, appears to be that recommended by Daguerre in the first instance. The plate is first placed in clean cold water, and then into a solution of the hyposulphite of soda, in which it is moved to and fro, until all the yellow colour is completely gone. It is then removed, placed in an inclined position, and boiled rain water, cooled so as to be just warm, poured over the plate in a continuous stream. Any drops of water which may remain on the plate, must be removed by forcibly blowing over it. The process is now completed, as far as preventing the further action of Light is concerned. The picture is formed of the finely divided silver, amalgamated with the mercury deposited over those parts on which the che