190 HISTORY OF PHOTOGRAPHY. 



amiss to relate, as it issued in a ne\v aud very pretty variety of the pliotograpbie 

 art. A solution of salt of extreuje dilution was mixed with nitrate of silver 

 so diluted as to form a liquid only slightly milky. This was poured into a 

 somewhat deep vessel, at the bottom of which lay horizontally a very clean 

 glass plate. After many days the greater part of the liquid was decanted off 

 with a siphon tube and the last portions very slowly and cautiously drained 

 away, drop by drop, by a siphon composed of a few fibers of hemp laid parallel 

 and moistened without twisting. The glass was not moved till quite dry and 

 was found coated with a pretty uniform film of chloride of silver of delicate 

 tenuity and chemical purity, which adhered with considerable force and was 

 very sensitive to light. On dropping on it a solution of nitrate of silver, how- 

 ever, and spreading it over by inclining the plate to and fro (which it bore 

 without discharging the film of chloride) it became highly sensitive, although 

 no organic matter could have been introduced with the nitrate, which was 

 quite pure, nor could any indeed have been present, unless it be supposed to 

 have emanated from the hempen filaments, which were barely in contact with 

 the edge of the glass and which were constantly abstracting matter from its 

 surface in place of inti'oduciug new. 



Exposed in this state to the focus of a camera with the glass toward the inci- 

 dent light it became impressed with a remarkably well-defined negative picture 

 which was direct or reversed, according as looked at from the front or the back. 

 On pouring over this cautiously, by means of a pipette, a solution of hyposul- 

 phite of soda the picture disappeared, but this was only while wet, for on wash- 

 ing in pure water and drying it was restored and assumed much the air oi' a 

 daguerreotype when laid on a black ground, and still more so when smoked at 

 the back, the silvered portions reflecting most light, so that its characters had in 

 fact changed from negative to positive. From such a picture (of course before 

 smoking) I have found it practicable to take photographic copies, and although 

 I did not in fact succeed in attempting to thicken the film of silver by con- 

 necting it under a weak solution of that metal with the reducing pole of a 

 voltaic pile, the attempt afforded distinct indications of its practicability with 

 patience and perseverance, as here and there over some small portions of the 

 surface the lights had assumed a full metallic brilliancy under this process. 

 T would only mention further to those who may think this experiment worth 

 repeating that all my attempts to secure a good result I)y drying the nitrate 

 in the film of chloride have failed, the crystallization of the salt disturbing 

 the uniformity of the coating. To obtain delicate pictures the plate must be 

 exposed wet, and when withdrawn must immediately be plunged into water. 

 The nitrate being thus abstracted, the plate may then be dried, in which state 

 it is half fixed, and it is then ready for the hyposulphite. Such details of 

 manipulation may appear minute, but they can not be dispensed with in prac- 

 tice, and cost a great deal of time and trouble to discover. 



This mode of coating glass witli films of precipitated argentine or other com- 

 pounds affords, it may be observed, the only effectual means of studying their 

 habitudes on exposure to light, free from the powerful and ever-varying influ- 

 ence of the size in paper and other materials used in its manufacture, and esti- 

 mating their degree of sensibility and other particulars of their deportment 

 under the influence of reagents. I find, for example, that glass so coated witli 

 the iodide of silver is much more sensitive than if similarly covered with tlie 

 chloride, and that if both be washed with one aud the same solution of nitrate 

 there is no comparison in respect of this valuable quality, the iodide being far 

 superior, and, of course, to be adopted in preference for the use of the camera. 

 It is, however, more difficult to fix, the action of the hyposulphites on this com- 

 pound of silver being comparatively slow and feeble. 



