When an image formed of a powdery substance has been obtained either by dusting (as described above), or by filling an engraved plate with the powder, the plate bearing the image is exposed to a vapor, which has no effect upon it. The powder alone absorbs the vapor, and if the plate be then applied to a surface coated with some substance capable of being acted upon by the vapor, an image is obtained upon this second surface. For example, the lines of an engraved copper-plate are filled with powdered albumen. On the other hand, a few drops of hydrofluoric acid are spread over a wooden board, and the powdered engraving is exposed for ten to fifteen seconds to the fumes disengaged by holding it about a quarter-of-an-inch above the board. The acid is absorbed by the powdered albumen without attacking the copper. If this plate be now placed in close contact with any surface (metal, paper, or glass) which has been covered with a coating of sugar and borax, and dried immediately, a deliquescent fluoborate of soda is produced under the action of the acid vapors, the sugar becomes tacky, and, by brushing a powder over this surface, the image appears immediately. In M. Davanne's opinion this new invention of M. Garnier's seems likely to have a useful and extended application. The image may be made with powder of any desired color. If it is on glass, it may be transferred to paper or other support by means of collodion or gelatine. By employing enamel powders, this process gives a new method of producing vitrified images. It may also be used as a simple method of reproducing engravings under certain circumstances; copies of diagrams, however intricate, could easily be produced on glass by it, and used for the illustration of lectures by means of the magic lantern.--_Photo. News._ * * * * * DANGERS OF PYROGALLIC ACID. By DR. T.L. PHIPSON. Some time ago, Dr. Napias, of Paris, who devotes much of his time to matters connected with hygiene, took up the subject of the hygiene of the photographer, and published in the _Moniteur de la Photographie_ a series of papers which were afterward translated into English and published by Messrs. Piper & Carter, of London. In them the worthy author has considered the action on the economy of the various poisonous substances which pass daily through the hands of our readers, and the best means of counteracting their influence.