218 , report — 1859. 



56. The only extra piece of apparatus required, is a pressure frame ; 

 which consists essentially of a stout piece of plate glass in a frame, with an 

 arrangement for screwing a flat board, the size of the glass, tight against it. 

 Though apparently very simple, some care is required, when the frame is a 

 large one, in arranging the screw and board at the back, so as to obtain an 

 equal pressure all over the surface ; unless this is done, the glass will be very 

 liable to break. The pressure frames supplied to us by Messrs. Newman 

 and Murray, 122 Regent Street, are unexceptionable in this respect. The 

 board should of course be well-padded with velvet, and the lateral dimensions 

 of the glass should be the same as those of the gutta-percha dishes (8). 



57. The extra chemicals required for this process are chloride of sodium 

 and chloride of gold. Generally speaking, for the former, common table- 

 salt will be found quite pure enough ; but as the quantity required is but 

 small, it will perhaps be found better to obtain some of the recrystallized 

 salt along with the other chemicals. 



The chloride of gold is merely required for an artistic effect. Many 

 persons object to the reddish-brown appearance of ordinary photographic 

 positives ; the addition of a little chloride of gold to the fixing bath converts 

 this into a rich brown or black ; the trifling quantity required removes any 

 objection to its use on the score of expense. 



58. I prefer using the same kind of paper for positives as for negatives 

 (20). Messrs. Canson manufacture a thicker paper, which is generally 

 called positive paper, but I think the thin is far preferable ; the surface is 

 smoother, and the various solutions penetrate much better. 



59. The first operation which the paper has to undergo is salting ; the 

 bath for this purpose consists of 



Chloride of sodium 100 grains. 



Distilled water 40 ounces. 



Filter this into a clean dish, and completely immerse the sheets, marked 

 as directed (27). This is best done by laying them gently on the surface of 

 the liquid, and then pressing them under by passing a glass rod over them ; as 

 many sheets as the dish will hold may be thus immersed one after the other. 

 Allow them to soak for about ten minutes, then lift and turn them over in a 

 body ; afterwards they may be hung up to dry (38), commencing with the 

 sheet which was first put in. When dry, they may be taken down and put 

 aside for use at any future time. The sheets in drying generally curl up 

 very much ; it will therefore be found convenient in the next process, if the 

 salted sheets, before being put away, have been allowed to remain in the 

 pressure frame, screwed tight, for about twenty.four hours. This makes 

 them perfectly flat. 



60. The exciting bath is composed of 



Nitrate of silver 150 grains. 



Distilled water 10 ounces. 



After filtering, pour the solution into a clean dish ; and then lay the sheets, 

 salted as above, on the surface, face downwards, gently breathing on the 

 back, if it be necessary, to counteract the tendency to curl up ; let them 

 remain on this bath for about ten minutes, and then hang up to dry (38). 



61. This exciting bath will serve for nearly one hundred sheets; it will 

 then be better to put it on one side (6i), and make a new bath. It is not 

 advisable to excite more positive sheets than will be likely to be required in 

 the course of a week, for they gradually turn brown by keeping, even in the 

 dark, and lose sensitiveness. They will, however, keep much better if 

 pressed tight in the pressure frame, and thus protected from the air. 



