A Manual Of Photography

70 HISTORY OF PHOTOGRAPHY. the details in a soft and pleasing style. Being then laid by in a drawer, with free access of air, the picture again faded, by very slow degrees, and on January 2, 1842, was found quite oblite- rated. Being then subjected to the acid vapour the colour was reproduced. " Viola odorata.—Chemists are familiar with the colour of this flower as a test of acids and alkalies, for which, however, it seems by no means better adapted than many others; less so, indeed, than that of the Violar tricolor, the common purple iris, and many others which might be named. It oflers, in fact, another and rather a striking instance of the simultaneous existence of two colouring ingredients in the same flower, com- porting themselves differently, not only in regard to light but to chemical agents. Extracted with alcohol, the juice of the violet is of a rich blue colour, which it imparts in high perfec- tion to paper. Exposed to sunshine, a portion of this colour gives way pretty readily, but a residual blue, rather inclining to greenish, resists obstinately, and requires a very much longer exposure (for whole weeks, indeed) for its destruction, which is not even then complete. Photographic impressions, therefore, taken on this paper, though very pretty, are exceedingly tedious in their preparation, if we would have the lights sharply made out. " Sparaxis tricolor ?, car.—Stimulating Effects of Alkalies.— Among a great many hybrid varieties of this genus, lately forwarded to me from the Cape, occurred one of a very intense purplish-brown colour, nearly black. The alcoholic extract of this flower in its liquid state is rich crimson-brown. Spread on paper, it imparted a dark olive-green colour, which proved per- fectly insensible to very prolonged action, either of sunshine or the spectrum. The addition of carbonate of soda changed the colour of this tincture to a good green, slightly inclining to olive, and which imparted the same tint to paper. In this state, to my surprise, it manifested rather a higli degree of photo- graphic sensibility, and gave very pretty pictures with a day or two of exposure to sunshine. When prepared with the fresh juice there is hardly any residual tint, but if the paper be kept, a great amount of indestructible yellow remains outstanding. The action is confined chiefly to the negative end of the spectrum ; all but the first five or six parts beyond the yellow show little more than a trace of action. A photograph impressed on this paper is reddened by muriatic acid fumes. If then transferred to an atmosphere of ammonia, and when super- saturated the excess of alkali allowed to exhale, it is fixed, and of a dark green colour. Botli the tint and sharpness of the picture, however, suffer in this process.