362. The instructions for working the platinotype process are given now in detail. A general description of the process is not necessary. It is in brief as follows: The sensitized paper, containing only salts of iron and platinum, is exposed under a negative or in the solar camera in the usual manner; it is then floated for two or three seconds on a hot solution of oxalate of potash; after this it is washed in a weak solution of acid, and finally in water.

Sensitizing. - This operation is a very important one, and upon it the success of the printing mostly depends. Failures are in most cases referable to errors in sensitizing. Sensitizing should be conducted in a room lighted by a yellow or very feeble white light, or by gas. For contact printing the sensitizer is made by dissolving sixty grains of the platinum salt in one ounce of the iron solution. To facilitate the solution, stir with a glass rod until all is dissolved. Use as soon as made, otherwise decomposition is likely to occur, especially in warm weather. As a rule, the solution will keep good for half an hour. Sensitizing solutions which have decomposed give flat prints with impure whites. For contact work on the smooth 18 x 22 inch paper, a little less than two drachms of the sensitizer is sufficient to coat a sheet. The paper should be placed face upwards on the special glass topped table and secured there by the side springs. A little less than two drachms of the sensitizer should be put on the middle of the sheet; it should then be spread over the surface in as even a manner as possible by the special squeegee. For contact work on the sized rough paper, the same quantity of the sensitizer may be employed, but the coating is better distributed by means of a small pad of flannel made soft by a tuft of cotton placed inside. As soon as sensitized, the paper should be hung up by two corners until the moisture has disappeared from its surface; this should take ten minutes; it should then be made perfectly dry by warming before a fire or stove. (It is of the utmost importance that the paper be made thoroughly dry.) Sufficient time but has also failed to meet the requirement of the every - day printer.

The slats of platinum have been tried end found of much service, end so directions are given below for the practice of his method with platinum, by Mr. W. Willis, Jr., the inventor and patentee.

should elapse between the sensitizing and drying. If the paper be dried too soon, some of the image will float off in the developing bath. On the other hand, if it be not dried, say within twelve minutes after sensitizing, there will frequently be a tendency to flatness, and the image will probably bo too much sunk in. When the air is very dry, it is necessary to create a moist atmosphere in the sensitizing room by watering the floor or walls, in order to prevent the paper from becoming too rapidly surface dry. A damp cupboard or damping-box may be used for this purpose. F<>r solar work and for prints to be finished in crayon, ink, water-color, or pastel, the sensitizer is made by dissolving forty grains of platinum salt in one ounce of the iron solution. Use at once. Three drachms of the sensitizer are sufficient to coat a 26x30 inch sheet. The paper should be placed on a plate of glass and held in position by clips. The sensitizer should be applied to the sheet, by a tuft of clean cotton wool, in as even a manner as possible. When the sheet has been sensitized, it should be allowed to become surface dry and then be perfectly dried before a fire or stove or in a hot drying cupboard, For solar work five to eight minutes are sufficient for drying. To prevent streaks and stains, the flannel should be removed from the squeegee as soon as sensitizing is completed, and washed in the clearing solution and then in water; flannel pads must be treated in like manner; tufts of cotton employed for solar work should be discarded and fresh ones taken every fifteen minutes; sensitizing tables must be kept perfectly clean.

Exposure. - The correct exposure is ascertained by inspection of the paper in a very weak light in the usual manner. As a general rule, the exposure is complete when the detail in the high-lights becomes faintly visible. As soon as exposed, the print should be placed in a tin can containing a little dry chloride of calcium, to preserve it from moisture until developed.

Development. - Development should be conducted in a feeble white light or gas-light. It may be proceeded with immediately after the print is exposed, or more conveniently at the end of the day's printing. The developer is made by dissolving one hundred and thirty grains of oxalate of potash in each ounce of water. A large quantity of this solution may be made up. It will keep indefinitely. The solution should bo made faintly acid by oxalic acid. Contact prints are developed by floating the printed surface for a few seconds on this developing solution, which is conveniently contained in a flat - bottom dish of enamelled boa or porcelain, supported on an iron tripod. A Bunsen burner, with rose-top to spread the flame, forms the best means for supplying the heat; or a spirit lamp may be used. A temperature varying between 170° and 180° F. is the standard temperature for the developer, For prints on rough paper, it is better to stir up the developer between each development to destroy any scum which may form. To develop large solar prints, a V-shaped earthenware or enamelled iron trough should be used. The developer should be heated in this trough by a row of small gas jets placed underneath, or by any other convenient device. The print is developed by being slowly and steadily drawn through the liquid at the bottom of the trough; it is held under the surface of the liquid by a heavy glass rod; this glass rod revolves as the print is drawn under it.

Clearing and Washing. - The developed prints must be washed in two baths of a weak solution of citric acid to clear them. This solution is made by dissolving one ounce of citric acid in thirty ounces of water. As soon as developed, the prints should he immersed face downwards in the first hath of acid, and after they have remained in it for about ten minutes they should he removed to the second bath, in which they should remain another ten minutes. The first hath may be used for two batches of prints. The second acid-bath must always be fresh. This bath may be used again as the first bath for a succeeding batch of prints. On no account should the prints be placed in plain water on leaving the developer. After the prints have passed through the two changes of acid, they should be rapidly rinsed, and then well washed in two or three changes of water during about half an hour. They are then finished. The object of this washing in acid and water is to remove the iron salt with which the paper is sensitized.