This very useful instrument is for ascertaining the strength of the silver nitrate solution, which becomes weakened to a certain extent, after the immersion of every plate. To use the argentometer, fill the glass jar to within about 2 in. of the top with the liquid to be tested, and then insert it; the degree on the scale that floats on a level with the surface of the fluid will indicate the number of grains of silver nitrate contained in each ounce of the solution. There must be sufficient liquid to prevent the argentometer resting on the bottom of the jar. For strengthening a bath to the required standard, it is generally found more convenient to have a stronger prepared bath to add to the weak one, than to add the silver nitrate direct.
During an easterly wind, double or treble the exposure is necessary in outdoor work. The windows of the dark room will require to be re-yellowed in the spring;. chemical darkness sufficient in the winter sun will be insufficient in spring and summer. An even temperature should be kept in the studio all the year round; in cold weather all the operations are tediously prolonged, unless the rooms are artificially warmed.