this sensitiveness when it does exist.[G] M. Regnault has laid before the Academy of Sciences, at Paris, some photographic specimens on paper, obtained by M. Blanquart-Evrard, by a modification of the usual process. In the preparations hitherto described, one part of the process presented serious difficulties, namely, that of the use of gallic acid in order to produce the impression. It happened frequently, that a proof taken in too mild a light, or of too large dimensions, could not receive the necessary force before disappearing, as it may be said, under the uniform colour produced by the mixture of the gallic acid with the aceto-azotate of silver, with which the paper is imbued. After having ascertained that the gallic acid produces this uniform colour on the impression, only because it is combined in small quantity with the aceto-azotate of silver, M. Blanquart-Evrard removes all the difficulty. After taking the proof from the camera obscura, he plunges it into a vessel of large dimensions, covered with a layer of one centimètre of gallic acid of cold saturation. The bath is agitated during the immersion; and the action may be thus prolonged until the impression has obtained the necessary force to secure a good result. The proof is then washed, and the gallic acid is replaced by a solution of bromure of potassium, or chloruret of sodium, in which it is left for about a quarter-of-an-hour.[H] The chromatype, discovered by Mr. Hunt, consists in washing good letter-paper with the following solution:-- Bi-chromate of potash 10 grains Sulphate of copper 20 grains Distilled water 1 ounce Papers prepared with this are of a pale yellow colour; they may be kept for any length of time without injury, and are always ready for use. For copying botanical specimens or engravings, nothing can be more beautiful. After the paper has been exposed to the influence of sunshine, with the objects to be copied superposed, it is washed over in the dark with a solution of nitrate of silver of moderate strength. As soon as this is done, a very vivid positive picture makes its appearance; and all the fixing these photographic pictures require is, well washing in pure water. M. Niepcé de St. Victor finds that, if a sheet of paper on which there is writing, printed characters, or a drawing, be exposed for a few minutes to the vapour of iodine, and there be applied immediately afterwards a coating of starch, moistened by slightly acidulated water, a faithful tracing of the writing, printing, or drawing, will be obtained. M. Niepcé has also discovered that a great number of substances, such as nitric acid, chlorurets of lime and mercury, act in a similar manner; and that various vapours, particularly those of ammonia, have the effect of vivifying the images which are obtained by photography.