ON SELF-RECORDING MAGNETOGRAPHS. 215 



absorption of the wax, or defect in the iodizing, and it will be as well to 

 reject sheets so marked. 



40. As soon as the sheets are quite dry, they can be put aside in a box 

 for use at a future time. There is a great deal of uncertainty as regards the 

 length of time the sheets may be kept in this state without spoiling ; I can 

 speak from experience as to there being no sensible deterioration after a 

 lapse of ten months, but further than this I have not tried. 



Up to this stage it is immaterial whether the operations have been per- 

 formed by daylight or not ; but the subsequent treatment, until the fixing 

 of the picture, must be done by yellow light (5). 



41. The next step consists in rendering the iodized paper sensitive to light. 

 Athough, when extreme care is taken in this operation, it is hardly of any 

 consequence when this is performed, yet in practice it will not be found 

 convenient to excite the paper earlier than about a fortnight before its being 

 required for use. The materials for the exciting bath are nitrate of silver, 

 glacial acetic acid, and water. Some operators replace the acetic acid by 

 tartaric acid ; but as I cannot perceive the effect of this change except in a 

 diminution of sensitiveness, I have not adopted it. It is of little importance 

 what be the strength of the solution of nitrate of silver; the disadvantages 

 of a weak solution are, that the sheets require to remain in contact with it 

 for a considerable time before the decomposition is effected, and the bath 

 requires oftener renewing ; while with a bath which is too strong, time is 

 equally lost in the long-continued washing requisite to enable the paper to 

 keep good for any length of time. The quantity of acetic acid is also of little 

 consequence. 



42. In the following bath, I have endeavoured so to adjust the proportion 

 of nitrate of silver, as to avoid as much as possible both the inconveniences 

 mentioned above : — 



Nitrate of silver 300 grains. 



Glacial acetic acid 2 drachms. 



Distilled water 20 ounces. 



The nitrate of silver and acetic acid are to be added to the water, and when 

 dissolved, filtered into a clean dish (10), taking care that the bottom of the 

 dish be flat, and that the liquid cover it to the depth of at least half an inch 

 all over; by the side of this, two similar dishes must be placed, each con- 

 taining distilled water. 



43. A sheet of iodized paper is to be taken by one end and gradually 

 lowered, the marked side downwards, on to the exciting solution, taking care 

 that no liquid gets on to the back, and no air-bubbles are enclosed. 



It will be necessary for the sheet to remain on this bath from five to ten 

 minutes ; but it can generally be known when the operation is completed by 

 the change in appearance, the pink colour entirely disappearing, and the sheet 

 assuming a pure homogeneous straw colour. When this is the case, one 

 corner of it must be raised up by the platinum spatula, lifted out of the dish 

 with rather a quick movement, allowed to drain for about half a minute, and 

 then floated on the surface of the water in the second dish, while another 

 iodized sheet is placed on the nitrate of silver solution ; when this has re- 

 mained on for a sufficient time, it must be in like manner transferred to the 

 dish of distilled water, having removed the previous sheet to the next dish. 



44. A third iodized sheet can now be excited, and when this is completed, 

 the one first excited must be rubbed perfectly dry between folds of clean 

 blotting-paper (14), wrapped up in clean paper, and preserved in a port- 

 folio until required for use ; and the others can be transferred a dish forward, 



