HISTORY OF PHOTOGRAPHY. l73 



gloomy place, a positive picture, the exact complement of the ■negative one on the 

 other side, though wanting of course iu sharpness if the paper be thick, slowly 

 and gradually makes its appearance there, and in half an hour acquires con- 

 siderable intensity. I ought to mention that the ferro-tartaric acid in question 

 is prepared by precipitating the ferro-tartrate of ammonia I)y acetate of lead 

 and decomposing the precipitate l»y dilute sul])huric acid. 



THE COLORING MATTER OF FLOWERS. 



The results obtained by Sir John Herschel on tlie coloring jnices of 

 flowers are too remarkable to be omitted in a treatise in which it is 

 desirable that every point should be registered up to the date of pub- 

 lication, which connects itself with the phenomena of chemical change 

 applied to photography : 



In operating on the colors of flowers I have usually proceeded as follows : 

 The petals of the fresh flowers, or rather such parts of them as possessed a 

 uniform tint, were crushed to a pulp in a marble mortar, either alone or with 

 addition of alcohol, and the juice expressed by squeezing the pulp in a clean 

 linen or cotton cloth. It was then spread on paper with a flat brush and dried 

 in the air without artificial heat, or, at most, with the gentle warmth which 

 rises in the ascending current of air fi'om an Arnott stove. If alcohol he not 

 added, the application on paper must be performed immediately, since exposure 

 to the air of ths juices of most fiowers (in some cases even but for a few 

 minxites) irrecoverably changes or destroys their color. If alcohol be present, 

 this change does not usually take place or is much retarded, for which reason, 

 as well as on account of certain facilities afforded by its admixture in procur- 

 ing an even tint (to be presently stated), this addition was commonly but not 

 always made. 



Most flowers give out their coloring matter readily enough either in alcohol 

 or water. Some, however, as the Escholzias and Calceolarias, refuse to do so, 

 and require the addition of alkalies ; others of acids, etc. When alcohol is 

 'added, it should, however, be observed that the tint is often apparently much 

 enfeebled or even discharged altogether, and that the tincture when spread on 

 paper does not reappear of its blue intensity till after complete drying. The 

 temporary destruction of the color of blue heartsease by alcohol is curious, nor 

 is it by any means a singular instance. In some, but in very few cases, it is 

 destroyed, so as neither to reappear on drying, nor to be capable of i-evival by 

 any means tried. And in all cases long keeping deteriorates the colors and alters 

 the qualities of the alcoholic tinctures themselves, so that they should always 

 be used as fresh as possible. 



If papers tinged with vegetable colors are intended to be t>reserved, they 

 must be kept perfectly dry and in darkness. A close tin vessel, the air of 

 which is dried by tjuicklime carefully inclosed in double paper bags well 

 pasted at the edges to prevent the dust escaping, is used for this purpose. 

 Moisture, as already mentioned, especially assisted by heat, destroys them for 

 the most part rapidly, though some, as the color of the Senecio splendens, resist 

 obstinately. Their destructibility by this agency, however, seems to bear no 

 distinct relation to their photographic jtmijerties. 



This is also the place to observe that the color of a flower is by no means 

 always or usually that which its expressed juice imparts to white pai)er. In 

 many cases the tints so imparted have no resemblance to the original hue. 

 Thus, to give only a few instances, the red damask rose, of that intense variety 



