In one of the upper rooms of the Electrical Exhibition in Paris, there is an interesting collection of plates and proofs produced by various methods of photo-engraving, invented by M. Henri Garnier, whose name is so well known in connection with these processes, and whose beautiful plate of the Chateau of Maintenon gained for him a gold medal at the Paris International Exhibition of 1867. Some interesting details of these processes are given in an extract from a report on them by M. Davanne to the Societe d'Encouragement pour l'Industrie Nationale, read at its sitting on the 22d July last, of which copies are distributed gratis in the exhibition. The report opens with a brief allusion to M. Garnier's continuous labors in permanent photographic printing, commencing with the ingenious mercury process worked out in conjunction with M. Salmon, and published in 1855, in which a print which has been exposed to the fumes of iodine is laid down on a plate of polished brass, so that the iodine, absorbed by the printed lines, slightly attacks the brass; mercury being then rubbed over the brass, forms an amalgam with the iodized parts. If a roller charged with printing ink be now passed over the plate, the ink will only be taken on the pure brass, and not on the iodized parts. The plate is next bitten with acid nitrate of silver, and may then be treated in various ways, so as to form either a printing-block or an engraved plate. The process never came to any practical use, but led M. Garnier to the invention of the very valuable and largely used process of acierage or steel-facing, by which the surface of engraved copper-plates is so hardened and protected by a thin coating of iron that instead of only a few hundred impressions, many thousands can be printed from a plate without the slightest deterioration. The next invention noticed is the citrate of iron process of M.M. Salmon and Garnier, in which a paper, coated with a sirupy solution of citrate of iron, is exposed to light under a positive print for a period varying from eight to ten minutes in the sun, to half or three-quarters of an hour in the shade. In the parts where the light has acted the paper becomes non-hygroscopic in proportion to the intensity of the action of the light upon it. The paper being left for a short time to absorb moisture from the air, is dusted over with lamp-black, which, attaching itself to the unexposed parts, reproduces an exact image of the original drawing. M. Garnier has since greatly modified this method of obtaining an image by dusting, and applied it to various processes of photo-engraving. The report then proceeds to give the following details of a process of photo-engraving, which was exhibited before the society by M. Garnier in March last: PHOTOGRAVURE. In photo-engraving a distinction must always be made between the reproduction of drawings in line and those with shaded tints.