214 REPORT — 1859. 



of a bromide, but in a less marked degree. As no particular advantage could 

 be traced to it, it was not employed. 



35. I have also tried most of the different forms of organic matter which 

 it is customary to add to this bath, but I cannot recommend them ; the 

 most that can be said is, that some of them do no harm. At first I thought 

 a little isinglass might be an improvement, as it instantly removes the greasi- 

 ness from the surface of the paper, and allows the iodide of potassium to 

 penetrate more readily. Unfortunately, however, it interferes with the most 

 important property of this process, that of remaining sensitive for a long time. 



36. I think the best results are obtained when the iodide and bromide 

 are mixed in the proportion of their atomic weights ; the strength being as 

 follows : — 



Iodide of potassium .... 582*5 grains. 

 Bromide of potassium .... 417*5 grains. 

 Distilled water 40 ounces*. 



When the two salts have dissolved in the water, the mixture should be 

 filtered ; the bath will then be fit for use. 



37. At first a slight difficulty will be felt in immersing the waxed sheets 

 in the liquid without enclosing air-bubbles, the greasy nature of the surface 

 causing the solution to run off. The best way is to hold the paper by one 

 end, and gradually to bring it down on to the liquid, commencing at the 

 other end ; the paper ought not to slant towards the surface of the bath, or 

 there will be danger of enclosing air-bubbles ; but while it is being laid 

 down, the part out of the liquid should be kept as nearly as possible per- 

 pendicular to the surface of the liquid ; any curling up of the sheet, when 

 first laid down, may be prevented by breathing on it gently. In about ten 

 minutes the sheet ought to be lifted up by one corner, and turned over in 

 the same manner ; a slight agitation of the dish will then throw the liquid 

 entirely over that sheet, and another can be treated in like manner. 



38. The sheets must remain soaking in this bath for about three hours ; 

 several times during that interval (and especially if there be many sheets 

 in the same bath) they ought to be moved about and turned over singly, 

 to allow of the liquid penetrating between them, and coming perfectly in 

 contact with every part of the surface. After they have soaked for a suffi- 

 cient time, the sheets should be taken out and hung up to dry ; this is con- 

 veniently effected by stretching a string across the room, and hooking the 

 papers on to this by means of a pin bent into the shape of the letter S. 

 After a sheet has been hung up for a few minutes, a piece of blotting-paper, 

 about one inch square, should be stuck to the bottom corner to absorb the 

 drop, and prevent its drying on the sheet, or it would cause a stain in the 

 picture. 



39. While the sheets are drying, they should be looked at occasionally, 

 and the way iu which the liquid on the surface dries, noticed ; if it collect 

 in drops all over the surface, it is a sign that the sheets have not been suffi- 

 ciently acted on by the iodizing bath, owing to their having been removed 

 from the latter too soon. The sheets will usually during drying assume a 

 dirty pink appearance, owing probably to the liberation of iodine by ozone 

 in the air, and its subsequent combination with the starch and wax in the 

 paper. This is by no means a bad sign, if the colour be at all uniform ; but 

 if it appear in patches and spots, it shows that there has been some irregular 



* While giving the above as the calculated quantities, I do not wish to insist upon their 

 being adhered to with any extreme accuracy. An error of a few grains on either side 

 would, I believe, be without any perceptible effect on the result. 



