Niepce, the original discoverer of the art in conjunction with Daguerre, used exclusively the bitumen of Judea ; this substance is changed by light, only with much slowness, yet irrespectively of the pictures taken in the camera, he succeeded in copying engravings by the sale action of the light, and in making others, trom which a limited number of impressions could be taken. He operated at first on tin plates, for which he afterwards substituted thin sheets plated with silver ; it was while endeavoring to strengthen the shades of his impressions on the plate that he used iodine. By this means he discovered the photogenic properties of the coating of iodide of silver, which are manif ested by a deep change of color, an unexpected result for the iodide of silver precipitated, is perhaps the insoluble compound of silver that darkens best ii. the light. To TAKE OUT STAINS FKOM THE HANDS— A correspondent gives the following directions for taking out stains on the hands of Da-guerreotypists :—Blue spots are produced by the union on the skin of a salt of iron with the cyanide of potassium. In this manner, unintentionally, Prussian blue is formed ; now Prussian blue is soluble in caustic alkalies, it can therefore be made to disappear by rubbing the dyed part with, a weak solution of potash or caustic soda ; ammonia likewise gets rid of it. Yellow spots are attributable to the formation of a sub-salt, or an oxide of iron. When recent they disappear more easily than when they have been allowed to remain for some time ; in the first case oxalic acid is useful, or the salt of sorrel ; in the second hydrochloric acid, diluted with two or three times its volume of water. Black marks may be of two kinds : if they are owing to the union of a salt of iron with gallic acid, which forms common ink ; they can be made to disappear with hydrochloric acid prepared as above. If they are owing to the action of a salt of silver on the gallic acid, by moistening them with hydrochloric acid, they can be classified in the list ot ordinary stains ot salts of silver. These latter always dye the skin black; in time this color changes to a violet, afterwards to a dark brown, to a light brown—and at last disappears. To get rid of these stains the employment of an alcoholic solution of iodine has been advised. This method olten efficacious, has the fault of dyeing the skin a yellow fawn color, the more disagreeable because it continues for several days. The infallible remedy is the cyanide of potassium. By spreading it in a powder over the part to be taken out, and then gently moistening it with water and rubbing it over the same, it will always clear off the stain Cyanide of potassium is a strong poison, it is therefore proper to prevent any harm that might result from its introduction undr the nails or in a scratch, to wash the hands afterwards with a little chlorine, or, preferably, Javelle water. The following is a resume of the directions to be employed :— Is;. Using hydrochloric acid, which destroys the yellow color, owing to the salts ot iron, and which restores all the salts ot silver to the state of chlorides. 2nd. Soda or any other caustic alkali which takes off the blue color attributable to Prussian blue, and neutralizes the little acid remaining on the skin after the former operation. 3rd. Cyanide of potassium, which takes away all the stain due to the salts of silver. 4th. Lastly, for sanitary precaution, chlri-ded or Javelle water.— [Lumire.
This article was originally published with the title "Daguerreotyping" in Scientific American 8, 26, 203 (March 1853)