SENSITISING.
The sensitising process is similar to that described in the paragraph " How to Sen sitise," p. 170; but there are certain ex ceptions which should be mentioned. The bath to be employed should be in a neutral condition, If acid, it will give poor flat prints.. The strength of the bath should be 50 grains per oz. for average negatives. The strength is governed by the contrast desired in the picture, and the solvent action liable with a weak solution of silver nitrate, as previously explained. For hard negatives, the solution may be allowed to fall to 30 grains per oz., whilst with weak negatives the strength of the bath may be raised to 80 grains per oz. It is not ad visable to tinker with the bath ; t•e best plan is to have the bulk of the solution made up for average negatives, and about 10 oz. of each of the varied solutions in addition, which will be sufficient for float ing paper in small pieces, say up to 12 in. by 10 in. or 15 in. by 12 in., if a dish with a perfectly flat bottom is used. Where only a small quantity of solution is to be used the shallower the dish is the better, as otherwise. the paper is a little difficult to get at. The reverse side of a glass
bottom dish may be used with care. To make up sufficient for general use, it is usual to dissolve 1,750 grains of pure re crystallised silver nitrate in 30 oz. of dis tilled water. Take care, in weighing it up, that the scale pans are scrupulously clean, or that a filter paper is placed in each into the solution ; if it turns blue the solu tion is alkaline, but if it remains red it is either acid or neutral : to ascertain which, tear a leaf from the blue litmus book, and immerse this in the solution ; if it is acid, I the paper will turn red, but if neutral it I will remain unchanged. Albumen paper is found to give the best results when not ' absolutely dry, and for this purpose it has been recommended (when working abroad in a very dry atmosphere, or in extremely hot weather in this country) to add 50 grains per oz. of some deliquescent salt, such as ammonium or sodium nitrate, which, crystallising out on the surface of scale, as any foreign matter in a neutral bath is sure to cause trouble.