THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 305 



materials employed by the manufacturers of porcelain; it should 

 also be flat, very thin, andsemitransparent ; if too thin, so that there 

 would be a chance of breaking, it may be attached by means of cement 

 to a piece of glass to give it strength. The substance of the plate 

 should be slightly porous, so as to enable it to imbibe and retain a 

 sufficient quantity of the chemical solutions employed. To prepare 

 the plate for use, it is first required to give it a coating of albumen, or 

 white of eggs, laid on very evenly, and then gently dried at a fire. 

 According as the plate is more or less porous, it requires more or less 

 of the albuminous coating; it is best to employ a very close-grained 

 porcelain, Avhich requires but little white of egg. The prepared 

 plate may be made sensitive to light in the same way in which a sheet 

 of paper is rendered sensitive; and we generally find the same 

 methods applicable for photographic pictures on paper applicable 

 to those on porcelain plates, and one of the processes employed by the 

 patentee is nearly the same as that patented by Mr. Talbot in 1841. 



The prepared plate is dipped into a solution of nitrate of silver 

 made by dissolving 25 grains of nitrate in 1 ounce of water; or the 

 solution is spread over the plate uniforml}^ with a brush. The plate 

 is then dried, afterwards dipi:)ed into a solution of iodide of potas- 

 sium of the strength of about 25 grains of iodide to 1 ounce of water, 

 again dried, and the surface rubbed clean and smooth with cotton. 

 The plate is now of a pale-yellow color, owing to the formation on 

 the surface of iodide of silver. . The plate, i^repared as above di- 

 rected, may be kept in this state until required, when it is to be ren- 

 dered sensitive to light by washing it over with a solution of gallo- 

 nitrate of silver, then placed in the camera ; and the image obtained 

 is to be rendered visible and sufHciently strengthened by another 

 washing of the same liquid, aided by gentle warmth. The negative 

 picture thus obtained is fixed by washing it with water, then Avith 

 bromide of potassium, or, what is still better, hyposulphite of soda, 

 and again several times in water. The plate of porcelain being semi- 

 transparent, positive pictures can be obtained from the above-men- 

 tioned negative ones by copying them in a copying frame. 



The picture obtained on porcelain can be altered or modified in 

 appearance by the application of a strong heat, a process not appli- 

 cable to pictures taken on paper. With respect to this part of their 

 invention, the patentees claim: 



The obtaining by means of a camera or copying frame photugrapliic in)ages 

 or pictures upon slabs or plates of porcelain. 



The second part relates to the process which has been discovered 



and improved upon by Mr. Malone, who is associated with Mr. Fox 



Talbot in the patent. The patentees' improvement is a method of 



obtaining more complete fixation of photographic pictures on paper, 



SM 1904 20 



