370 SCIENTIFIC AMUSEMENTS. 



salt solution ; in either case care must be taken that no air bubbles 

 adhere to the surface of the paper, as these will produce spots in 

 the finished print.* 



The dry salted paper can be kept for a long time unaltered ; 

 when required for use it is floated on a solution of nitrate of silver 

 (1 part nitrate to 8 or 10 of distilled water), care being again taken 

 to avoid adherence of bubbles ; after remaining in contact with 

 the silver solution for about five minutes it is removed and hung 

 up to dry as before. This sensitising operation and the subsequent 

 drying must be performed in a room into which no daylight enters, 

 a candle or small gas flame surrounded by yellow semi-transparent 

 paper, or preferably a yellow glazed lantern, being employed as 

 source of light. When paper salted only on one side is used, that 

 side must be the one in contact with the silver solution. 



Fig. 201. Printing Frame. 



If the print is to be taken from a glass negative, the latter 

 (duly fixed, dried, and varnished with a thin film of transparent 

 varnish to prevent rubbing of the surface) is placed in a wooden 

 frame like a transparent slate, with a removable back kept in 

 position by a spring clip, &c., the sensitised paper being next to 

 it, with the sensitive side nearest the glass. Fig. 201 represents 

 one form of printing frame. The whole is then exposed to day- 



* " Albuminised paper," generally employed for cartes de visite, &c., is a 

 superior kind of paper, coated on one side with albumen (prepared white of 

 egg) by floating it on a strong solution of that substance to which some 

 chloride of ammonium or other salt capable of acting on nitrate of silver has 

 been added. Such paper consequently requires no further salting before ; use. 



