346 Mr, T. A. Malone, on the Application of Light [Feb. 13, 



worthy of further investigation. Here the image is truly drawn by 

 light and engraved by electricity. 



M. Fizeau, about the year 1844, also patented in this country, 

 in conjunction with M. Claudet, a process for engraving the da- 

 guerreotype image. The speaker was instructed in this process by 

 M. Fizeau, and worked for many months at its perfection. Kesults 

 obtained both in France and England were upon the table, and 

 showed that in cases where great delicacy of delineation was re- 

 quired, as in certain anatomical subjects, this process had not been 

 surpassed. It quite justified the formation of a second division of 

 the available photographic engraving processes. 



M. Fizeau, like Mr. Grove, availed himself of the affinity of 

 chlorine for silver, but relied on chemical action for its application. 

 He (^M. Fizeau) made a solution of common salt and nitrite of 

 potash in water, to which he added nitric acid. This mixed acid 

 acted immediately when aided by warmth, upon the silver of Da- 

 guerre's plate, and left untouched the parts supposed to be com- 

 pletely covered by mercury. Chloride of silver was thus ^t once 

 formed in the shadows of the images, and after some time in the 

 half tints also. A very faint etching was thus produced. A pro- 

 longed application of the acid would not further deepen the etching, 

 since the insoluble chloride of silver at first formed protected the 

 faintly etched parts from a further deepening corrosion. It was 

 therefore necessary to remove the chloride of silver by washing 

 with a solution of ammonia. This effected, the plate was ready for 

 a second application of the acid, when chloride of silver would be 

 again formed, to be once more removed by ammonia ; and this 

 alternation of solutions could be repeated a certain number of times, 

 the etching increasing in depth at each operation. But in practice 

 it was found that after a few applications of the acid the lights of 

 the image also gave way, and thus the engraving came to an untimely 

 end. To remedy this circumstance was M. Fizeau's great aim ; and 

 he succeeded in a marked degree by heating the etched plate in a 

 strong and boiling solution of caustic potash, after wiiich treatment 

 the lights resisted well the injurious action they had before suffered 

 from. It is not clear how the potash acts. . M. Fizeau has sup- 

 posed, and the speaker was inclined to support the view, that the 

 potash acts merely as a hot bath, possessing a proper and a regular 

 temperature which might restore the continuity of the amalgamated 

 surface of mercury and silver as often as it was weakened to the 

 point of breaking by the under-biting of the acid liquid. The 

 heating in potash is an important feature in M. Fizeau's i)rocess. 

 As soon as the etching has been carried as far as possible by the 

 acid mixture, the plate is dried and inked with fine printer's ink, 

 and an impression may be immediately taken ; but M. P^izeau 

 prefers that the ink should be allowed to dry in the hollows of the 

 plate, the unetched parts being wiped clean, so that gold may be 

 deposited only upon tlie bright parts by the electrotype process. 



