HISTOEY OF PHOTOGKAPHY. 165 



by washing in water, their color becomes a pure Prussian blue, which deepens 

 much by Iveeping. If the solution l)e mixed, there results a very dark violet- 

 colored ink, which may be kept uninjured in an opaque bottle, and will readily 

 furnish by a single wash at a moment's notice the positive paper in question, 

 which is most sensitive when wet. 



It seems at first sight natural to refer these curious and complex changes to 

 the instability of the cyanic compounds, and that this opinion is to a certain 

 extent correct is proved by the photographic impressions obtained on ijapers 

 to which no iron has been added beyond what exists in the ferrocyanic salts 

 themselves. Nevertheless, the following experiments abundantly prove that in 

 several of the changes above described the immediate action of the solar rays 

 is not exerted on these salts, but on the iron contained in the ferruginous solu- 

 tion added to them, which it deoxidizes or otherwise alters, thereby presenting 

 it to the ferrocyanic salts in such a form as to precipitate the acids in combi- 

 nation with the peroxide or protoxide cfT iron, as the case may be. To make 

 this evident, all that is necessary is simply to leave out the ferrocyanate in the 

 preparation of the paper, which thus becomes reduced to a simple washing 

 over with the ammonia-citric solution. Paper so washed is of a bright yellow 

 color and is apparently little, but in reality highly, sensitive to photographic 

 action. Exiwsed to strong sunshine, for some time indeed, its bright yellow 

 tint is dulled into an ochery hue or even to gray, but the change altogether 

 amounts to a moderate percentage of the total light reflected and in short 

 exiwsures is such as would easily escape notice. Nevertheless, if a slip of this 

 paper be held for only four or five seconds in the sun (the effect of which is 

 (luite imperceptible to the eye, and, when withdrawn into the shade, be washed 

 over with the feri'osesquicyanate of potash, a considerable deix)sit of Prussian 

 blue takes place on the part sunned and none whatever on the rest, so that on 

 w'ashing the whole with water a pretty strong blue impression is left, demon- 

 strating the reduction of iron in that portion of the paper to the state of 

 protoxide. The effect in question is not, it should be observed, peculiar to the 

 ammonio-citrate of iron. The ammonio and potassia-tartrate fully possess and 

 the perchloride, exactly veiitraliscfL partakes of the same property, but the 

 experiment is far more neatly made and succeeds better with the other salts. 



Ill further developiiieiit of these most interesting processes Sir 

 John Herschel says: 



The varieties of cyanotype processes seem to be innumerable, but that whicii 

 I shall now describe deserves particular notice, not only for its preeminent 

 lieauty while in progress, but as illustrating the peculiar power of the 

 ummoniacal and other persalts of iron above mentioned to receive a latent 

 picture, susceptible of development by a great variety of stimuli. This process 

 consists in simply passing over the ammonla-citrated paper on which such a 

 latent picture has been impressed, very sparingly and evenly, a wash of the 

 solution of the common yellow ferrocyanate (prussiate) of potash. The latent 

 picture, if not so faint as to be quite invisible (and for this purpose should not 

 be so), is negative. As soon as the liijuid is applied, which can not be in too 

 tlnn a film, the negative picture vanishes, and by very slow degrees is replaced 

 liy a positive one of a violet-blue color on a greenish-yellow ground, which at 

 a certain moment possesses a high degree of sharpness and singular beauty and 

 delicacy of tint. If at this Instant it be thrown into water, it passes imme- 

 diately to Prussian blue, losing at the same time, how'ever, much of its sharp- 

 ness, and sometimes, indeed, becoming quite blotty and confused. But if this 

 1)6 delayed, the picture, after attaining a certain maximum of distinctness, 



