190 ANNUAL OE SCIENTIFIC DISCOVERY. 



succeded, after patient perseverance, in discovering the following pro- 

 cess, which enabled him to do so : The steel plate, after having been 

 slightly unpolished by a mixture of acetic acid and sulphuric acid, is 

 first covered with a good coating of gelatine, holding in solution a red 

 salt, called bichromate of potassium, whose sensibility to light has long 

 been familiar to chemists. When this coating has been suitably laid 

 over the surface of the metal, and dried there, by the application of a 

 gentle heat, the plate seems of a uniform yellow color ; then the opera- 

 tor applies to its surface either the object whose mere silhouette is 

 sought, or a photographic drawing already obtained on paper by the 

 usual processes. For greater simplicity, suppose that the operator 

 wishes to obtain the impression of some vegetable organ, profoundly 

 marked a leaf of fern, for example. This leaf is applied to the plate, 

 and pressed by a piece of clear glass, after which, and while so pressed, 

 it is exposed to the sun for one or two minutes. The parts of the 

 plate exposed to the sun will be observed to become brownish ; and 

 when the leaf which protects the parts beneath it is removed, its image 

 will be found very clearly traced, by the preservation of the original 

 color of that part of the plate. This image, formed by application, be- 

 comes still more apparent when the plate is plunged in water, as the 

 colored salt is only dissolved there, where it has not been altered by 

 light. And the water also removing a certain quantity of the gelatine, 

 prepares at the same time the plate for the reactive which is to eat in- 

 to the steel. It was no easy task to select this reactive. Should the 

 operator pour on the plate nitric acid, commonly used by engravers, 

 and which Dr. Donne used the first time he sought to operate by re- 

 actives on metallic plates ? It commenced to act at once on the white 

 parts of the plate, but the gas disengaged removed the gelatine coat- 

 ing, and the acid soon ceased !o respect the contours traced by the 

 light, and after a while began to aifect the whole surface of the plate. 

 It was consequently necessary to resort to a reactive capable of acting 

 without disengaging gas, and whose action would not extend to those 

 parts which had been exposed to the sun. Fortunately there is a re- 

 active combining all these requisites ; it is a solution of bi-chloride of 

 platina. This reactive is not so expensive as its name would seem to 

 indicate ; for it is not the platina which is expended in this operation 

 but rather the chlorine which attacks the steel, and eats into it so 

 deeply as that the traces are able to take and retain a sufficient quan- 

 tity of printing-ink. So the operator pours upon the plate, placed 

 horizontally, a certain quantity of bi-chloride of platina ; he will ob- 

 serve the action commence immediately, and when he thinks it has 

 gone far enough, he pours it off the plate, and he will find that he has 

 the same quantity of platina as before the operation commenced. He 

 has nothing to do further than to clean the plate, by moderately rub- 

 bing it with some soft body, to be certain that it is really engraved, 

 and then he may send it to the printer. Mr. Talbot has sent to the 

 Academy, as yet, only proofs of tissues and objects of natural history, 

 obtained by simple application. They show that the corrosive action 

 which engenders the engraving follows very faithfully the contours 



