HISTORY OF PHOTOGRAPHY. 175 



half an hour is completely whitened. The color seems to resist the first impres- 

 sion of the light, as if by some remains of vitality, which, being overcome, the 

 tint gives way at once, and the discoloration when commenced goes on rapidly. 

 It does not even cease in the dark when once begun. Hence it happens that 

 photographic imi)ressions taken on such paper, which when fresh are very 

 sharp and beautiful, fade by keeping, visibly from day to day, however carefully 

 preserved from light. They require from half an hour to an hour to complete, 

 according to the sunshine. Hydriodate of potash cautiously applied retards 

 considerably, but does not untimately prevent this spontaneous discharge. 



Common ten iceeks' stocks; Mathiola annua. — Paper stained with the tinc- 

 ture of this flower is changed to a vivid scarlet by acids and to green by alka- 

 lies. If ammonia be used, the red color is restored as the ammonia evaporates, 

 proving the absence of any acid quality in the coloring matter sufficiently ener- 

 getic to coerce the elastic force of the alkaline gas. Sulphurous acid whitens 

 it, as does the alkaline sulphites, but this effect is transient, and the red color 

 is slowly restored by free exposure to air, especially with the aid of light, 

 whose influence in this case is the more remarkable, being exactly the reverse 

 of its ordinary action on this coloring principle, which it destroys irrecoverably, 

 as above stated. The following experiments were made to trace and illustrate 

 this curious change : 



Two photographic copies of engravings taken on paper tinted with this color 

 were placed in a jar of sulphurous-acid gas, by which they were completely 

 whitened and all traces of the pictures obliterated. They were then exposed to 

 free air, the one in the dark, the other in sunshine. Both recovered, but the 

 former much more slowly than the latter. The restoration of the picture 

 exposed to the sun was completed in twenty-four hours, that in the dark not 

 till after a lapse of two or three days. 



A slip of the stained paper was wetted with liquid sulphurous acid and laid 

 on blotting paper similarly wetted. Being then crossed with a strip of black 

 paper, it was laid between glass plates and (evaporation of the acid being thus 

 prevented) was exposed to full sunshine. After some time the red color (in 

 spite of the presence of the acid) was considerably restored in the portion 

 exix)sed, while the whole of the portion covered by the black paper remained, 

 of course, perfectly white. 



Slips of paper stained as above were placed under a receiver beside a small 

 capsule of liquid sulphurous acid. When completely discolored they were sub- 

 jected (on various occasions and after various lengths of exposure to the acid 

 fumes, from half an hour to many days) to the action of the spectrum, and it 

 was found, as, indeed, I had expected, that the restoration of color urns oper- 

 ated, bjj rails complementary to those 'which destroy it in the natural state of 

 the paper, the violet rays being chiefly active, the blue almost equally so, the 

 green little, and the yellow, orange, and most refrangible red not at all. In one 

 experiment a pretty well defined red solar ^image was developed by the least 

 refrangible red rays also, being precisely those for which, in the unprepared 

 paper, the discoloring action is abruptly cut off. But this spot I never suc- 

 ceeded in reproducing; and it ought also to be mentioned that, according to 

 differences in the preparation not obvious, the degree of sensibility, generally. 

 of the bleached paper to the restorative action of light differed greatly, in some 

 cases a perceptible reddening being produced in ten seconds and a considerable 

 streak in two minutes, while in others a very long time was required to produce 

 any effect. The dormancy of this coloring principle under the influence of sul- 

 phurous acid is well shown by dropping a little weak sulphuric acid on the 

 paper bleached by that gas, which immediately restores the red color in all its 



