858 
the areas of flaking corresponding to them, we found as the most 
favourable conditions boiling of the pieces of the plate of 15 em’, 
resp. 65 cm’ with nitric acid of 50°/, for 60 min, resp. 90 min. 
The influence of the gelatine or of its reaction products then dis- 
appears at 8°/,') for the pieces of 15 cm’, and at 17 °/,*) for those 
of 65 cm’. 
The content of the silver nitrate solutions thus obtained from the 
photographic plate, can however not be read from the above described 
curve of gauging. For this purpose we must first make comparable 
silver-nitrate solutions of known content. This is done by developing, 
fixing, and rinsing unilluminated photographic plates of the same 
kind as has been used, in the same way as the blackened photo- 
graphic plates for the estimation of silver of which they must serve. 
Then pieces of 15 em?’ resp. 65 cm? of these unilluminated plates 
are heated with the same quantity of nitric acid and a known 
quantity of silvernitrate for 60 minutes resp. 90 minutes at 100°. 
The silver solutions thus obtained contain, therefore, the gelatine and 
its reaction products in the same form as the silver nitrate solutions 
obtained from the blackened plates, hence they can serve for the 
construction of the curves of gauging. 
Fig. 4 gives two of the curves of gauging used; the area of 
flaking in cm? has been plotted as ordinate against the number of 
mg. AgNO, in 14 em? of solution as abscissa; / serves in the deter- 
mination of the quantity of silver in plates of 15 em’, // in those 
of 65 cm?. As was to be expected, / lies higher than // on account 
of the smaller influence of the gelatine. That with zero silver a 
small value is still found for the area of flaking (cf. fig. 4 pieces 
AB 8 times enlarged), is a consequence of the turbidity through 
small particles of dust, which are raised by the stirring after the 
addition of the sodium chloride. The method has been used for 
silver solutions with a silver content of 0.5 x 10-3 to 156 x 10-3 
grams per liter; the sensitiveness of the method to detect traces of 
silver extends, however, much further than 0.5 « 107% grams of Ag 
per liter. 
) The circumstances made it necessary for us to manufacture the dishes 
ourselves. In this we met with the difficulty that all the well-known acid-proof 
cementing substances were attacked by the acid after shorter or longer time. 
At last we found a suitable cement in bakelite. When the recipe of BERTRAND— 
GAUTHIER is applied, the influence of the gelatine can be quite eliminated, 
hence for 100°. The acid-concentration required for this, however, attacks 
also the bakelite in less than an hour. 
2) More prolonged heating — which would enable us to carry up the 
percentage higher — met with practical difficulties. 
