HISTORY OF PHOTOGRAPHY. 1(>9 



and third, it must be present in a certain considerable quantity, wliicb quan- 

 tity must be greater tlie greater the amount of free acid present in the chloride. 

 I'nder these conditions the gold is precipitated by light as a black powder if the 

 liquid be in any bulk, and if merely washed over paper a stain is produced, 

 which, however feeble at first, under a certain dosage of the chloride, oxalate, 

 and free acid, goes on increaseing from day to day and from week to week, 

 when laid by in the dark, and especially in a damp atmosphere, till it acquires 

 almost the blackness of ink, the unsunned portion of the paper remaining un- 

 affected, or so slightly as to render it almost certain that what little action of 

 the kind exists is due to the effect of casual dispersed light incident in the 

 preparation of the pai)er. I have before me a specimen of paper so treated in 

 which the effect of thirty seconds' exposure to sunshine was quite invisible at 

 first and which is now of so intense a purple as may well be called black, while 

 the unsunned portion has acquired comparatively but a very slight brown. 

 And (which is not a little remarkable and indicates that in the time of expos- 

 ure mentioned the niaj-hnnm of effect was attained) other portions of the same 

 paper exposed in graduated progression for longer times, viz, one minute, two 

 minutes, and three minutes, ai-e not yi the least perceptible degree darker than 

 the portion on which the light had acted during thirty seconds only. 



If paper prepared as altove recommended for the chiysotype, either with the 

 ammonio-citrate or ammonio-tartrate of iron, and impressed, as in that process, 

 with a latent picture, be washed with nitrate of silver instead of a solution of 

 gold, a very sharp and beautiful picture is developed of great intensity. Its 

 disclosure is not instantaneous ; a few moments elapse without apparent effect. 

 The dark shades are then first touched in, and by degrees the details appear, 

 but much more slowly than in the case of gold. In two or three minutes, 

 however, the maximum of distinctness will not fail to be attained. The picture 

 may be' fixed by the hyposulphite of soda, which alone. I believe, can be fully 

 depended on for fixing argentine photographs. 



The best process for fixing any of the iihotograj^hs prepared with gold is as 

 follows : As soon as the picture is satisfactorily brought out I\y the auriferous 

 liquid it is to be rinsed in spring water, which must be three times renewed, 

 letting it remain in the third water five or ten minutes. It is then to I)e blotted 

 off and dried, after which it is to be washed on both sides with a somewhat 

 .weak solution of hydriodate of potash (iodide of potassium). If there be any 

 free chloride of gold present in the pores of paper, it will be discolored, the 

 . lights- passing to a ruddy brown ; but they speedily whiten again spontaneously, 

 or at all events on throwing it (after lying a minute or two) into fresh water, 

 in which, being again rinsed and dried, it is now perfectly fixed. 



PHOTOGRAPHIC PROPERTIES OF MERCURY. 



As an agent in the daguerreotype process, it is not. strictly speaking, photo- 

 graphically affected. It operates there oidy in virtue of its readiness to 

 amalgamate with silver properly prepared to receive it. That it possesses 

 direct photographic susceptibility, however, in a very eminent degree, is proved 

 by the following experiment : Let a paper be washed over with a weak solution 

 of periodide of iron, and, when dry, with a solution of proto-nitrate of mer- 

 cury. A bright yellow paper is produced, which (if the right strength of the 

 liquids be hit) is exceedingly sensitive while wet, darkening to a brown color 

 in a very few seconds in the sunshine. Withdrawn, the impression fades 

 rapidly, and the paper in a few hours recovers its original color. In operating 

 this change of color the whole si)ectrum is affective, with the exception of the 

 thermic rays beyond the red. 

 SM 1905 15 



