ON SELF-RECORDING MAGNETOGRAPHS. 221 



be put through the press in one night. The hot-pressing apparatus is used 

 by the paper-makers, and by some of the wholesale stationers. 



2. The iodizing bath, which should be kept in the dark when not in use, 

 consists of — 



Iodide of potassium 582^ grains. 



Bromide of potassium. . . . 417f grains. 



Distilled water 40 ozs. 



Iodine — sufficient to give the solution a decidedly red 

 tinge. 



With every fresh batch of paper, a small quantity of iodine should be 

 added to restore the red tone of the bath. 



The paper is to be hung up to dry in a dark cupboard, and, when dry, it 

 should be of a light reddish-brown colour ; if a deep red or purple, it will 

 want sensitiveness ; if nearly white, it will want keeping properties, and will 

 become discoloured during development. 



3. The exciting bath contains, — 



Nitrate of silver 750 gr9. 



Distilled water 30 ozs. 



Acetic acid 3 drins. 



A strong solution of nitrate of silver (100 grs. to 1 oz. of water) is kept 

 in a separate bottle for replenishing the exciting bath, which loses by use 

 both in quantity and strength. 2 drms. of this solution with \ drm. of acetic 

 acid, is added after exciting every three sheets (300 square inches) of paper. 

 The addition of acetic acid prevents discoloration during development, but 

 at the same time slightly diminishes the sensitiveness, and, if added in excess, 

 the intensity of the image is much weakened. When the bath is more than 

 a fortnight old, it is necessary to filter before using. With a weak and old 

 exciting bath the iodide of silver is apt to fall from the sheet in flakes while 

 in the bath, and the portions of the sheet so deprived of silver are no longer 

 sensitive to light : however, there need be no fear of this occurring while the 

 strength of the bath is maintained as above directed. The same exciting 

 solution has been used as long as three months with satisfactory results 

 (1000 square inches of paper being sensitized weekly). 



4. (Art. 44.) Instead of drying the sheets between blotting-paper, it has 

 been found to give cleaner and more uniform pictures to hang them up to 

 dry in a dark cupboard ; about an inch is cut off each end of the sheet and 

 rejected where the fingers have touched it, and where the fluid has accumu- 

 lated in dripping. 



5. It is very desirable that the exciting and fixing operations should be 

 performed at different times; for if, after fixing, the hands have not been care- 

 fully washed, the least remnant of fixing solution left upon the fingers is 

 communicated to the edge and dispersed over the moist surface of the newly 

 sensitized sheet, producing a stain which appears on developing. If a series 

 of black spots, proceeding from one corner of the sheet, show themselves 

 while developing, the cause should probably be looked for in the exciting 

 operation — a drop of the solution accidentally got on the upper side of the 

 floating sheet having trickled down when the sheet was held vertically : when 

 this occurs, it is better (instead of merely floating) to immerse the sheet in the 

 two washing dishes (see art. 43). 



6. A sheet of plate-glass, 20 inches by 18 inches, ground at the edges to 

 prevent the solution from flowing off, is used for developing. This was pro- 

 posed by Mr. Welsh, and it is found to answer extremely well : it rests upon 



