WEIGHING.
This must be done with extreme accur acy. The rough sort of weighing practised by many photographers is only likely to lead to disaster. A chemical balance, the pattern shown in either Fig. 12S or Fig. 129 being quite suitable for this purpose, should be employed in prefer ence to those in general use. It need not, however, possess the extreme deli cacy of adjustment of the best of such instruments. Place filter paper on the pan of each scale. Having weighed out the sub stances, place Vach aside on a separate round rapidly and drain off ; this will get rid of any dust, otherwise liable to adhe-e to it. The silver nitrate is then added to oz. of water in a boiling tube, and the solution warmed until dissolved. The boiling tube is a test tube of rather large bore used for boiling solutions. as shown in Fig. 12S. A strip of blotting paper, folded in four, is wrapped round it to form a handle. The potassinm bromide will readily dissolve in cold water. The gela tine should now be covered with 1 oz. of
water and allowed to swell for a few: minutes, then placed on the water bath' paper ; be careful not. to get them mixed, although there should be no fear of doing so, except perhaps in the ease of the gelatine, as the silver nitrate and potas sium bromide are easily distinguishable ciystals, the former occurring in flakes and the latter in cubes. In using a balance of the above kind, the substance should be placed upon the scales and then the arm gently raised. If the pointer indicates too much or too little, the arm should be lowered before the alteration is made. The 240 grains need not be weighed up at the present time. The other gelatine should be of a soft variety, Nelson's No. 1 being very suitable. Silver nitrate should recrystallised and absolutely pure, while the same remark applies to the potassium bromide ; needless to say, dis tilled water should be used throughout the operation.