Some very ingenious experiments have been made by Mr. Malone, from whose communication the following remarks are quoted :-
" To the white of an egg its own bulk of water is to be added; the mixture, beaten with a fork, is then strained through a piece of linen cloth, and preserved for use in a glass stoppered bottle; then a piece of plate glass, cleaned with a solution of caustic potash, or any other alkali, is to be washed with water and dried with a cloth. When the glass is about to be used, breathe on it, and rub its surface with clean new blotting paper; then, to remove the dust and fibres which remain, use cottonwool, or a piece of new linen. Unless this latter, and, indeed, every other precaution, is taken to prevent the presence of dust, the picture will be full of spots, produced by a greater absorption of iodine (in a subsequent process) in those than in the surrounding parts.
"On the clear glass pour the albumen, inclining the plate from side to side until it is covered; allow the excess to run off at one end of the corners, keeping the plate inclined, but nearly vertical. As soon as the albumen ceases to drop rapidly, breathe on or warm the lower half of the plate; the warmth and moisture of the breath will soon cause it to part with more of its albumen, which has now become more fluid : of course, care must be taken to warm only the lower half. "Wiping the edges constantly hastens the operation. Until this plan was adopted, the coatings were seldom uniform J the upper half of the plate retained less than the lower. When no more albumen runs down, dry the plate by a lamp, or by a common fire, if the dust that it is inclined to impart be avoided.
" The next operation is to iodize the plate. Dilute pure iodine with dry white sand in a mortar, using about equal parts of each; put this mixture into a square vessel, and place over it the albuminized plate, previously heated to about 100° Fah. As soon as the film has become yellow in colour, resembling beautifully stained glass, remove the plate into a room lighted by a candle, or through any yellow transparent substance, yellow calico for instance, and plunge it vertically and rapidly into a deep narrow vessel containing a solution of one hundred grains of nitrate of silver to fifty minims of glacial acetic acid, diluted with five ounces of distilled water. Allow it to remain until the transparent yellow tint disappears, to be succeeded by a milky-looking film of iodide of silver. Washing with distilled water leaves the plate ready for the camera.
" It may be here noted that the plate is heated in iodizing for the purpose of accelerating the absorption of the iodine : an exposure to the vapour for ten minutes, with a few seconds' immersion in the silver solution, has been found to be sufficient".
Hydrochloric acid, chlorine or bromine, may be used with the iodine to give increased sensibility to the plate.
The plate is removed from the camera, and we pour over it a saturated solution of gallic acid. A negative Calotype image is the result. At this point previous experimentalists have stopped. We have gone further, and find that by pouring upon the surface of the reddish brown negative image, during its development, a strong solution of nitrate of silver, a remarkable effect is produced. The brown image deepens in intensity until it becomes black. Another change commences—the image begins to grow lighter; and finally, by perfectly natural magic, black is converted into white, presenting the curious phenomenon of the change of a Talbotype negative into apparently a positive Daguerreotype, the positive still retaining its negative properties when viewed by transmitted light.
To fix the picture, a solution of one part of hyposulphite of soda in sixteen parts of water is poured upon the plate, and left for several minutes, until the iodide of silver has been dissolved. Washing in water completes the process.
" The phenomenon of the Daguerreotype," says Mr. Malone, " is in this case produced by very opposite agency, no mercury being present; metallic silver here producing the lights, while in the Daguerreotype it produces the shadows of the picture. We at first hesitated about assigning a cause for the dull white granular deposit which forms the image, judging it to be due simply to molecular arrangement. Later experiments, however, have given us continuous films of bright metallic silver, and we find the dull deposit becomes brilliant and metallic when burnished. It should be observed that the positive image we speak of is on glass, strictly analogous to the Daguerreotype. It is positive when viewed at any angle but that which enables it to reflect the light of the ray. This is one of its characteristics. It must not be confounded with the continuous film image which is seen properly only at one angle; the angle at which the other ceases to exist. It is also curious to observe the details of the image, absent when the plate is viewed negatively by transmitted light, appear when viewed positively by reflected light".