In steeping smaller objects before examination the whole of the water should be well stirred with a glass rod or poured two or three times from one vessel into another: 100, 50 or 25 cubic centimetres are then poured into a graduated glass or drawn up into a pipette. The water should be drawn up by suction slightly above the level of the mark upon the stem of the pipette, the upper end of which is immediately closed with the thumb. By slightly raising the thumb the water is allowed to run off until its upper surface is exactly level with the mark. The amount taken is then placed into a beaker (for 100 cubic centimetres a beaker of 400 c.c. capacity should be used), and, after the addition of a few drops of a solution of potassium chromate, is examined by titration. In the treatment of large objects, for which tubs are required, the necessary quantity of water may be drawn by means of a long pipette from the bottom of the tub, where the quantity of salt is always greatest, or through the tap at the bottom of the tub (as was done with the blocks from the Meten Chamber, in which case about 1 litre was drawn off into a glass out of which 100 c.c. were taken for titration). To obtain results which are comparable, care must be taken that the object is always as nearly as possible in the same quantity of water. After placing the larger blocks in the water, one examination should be made during the first few days, when the titration may require 20 c.c. or more of silver solution. There is no need to examine for chlorine while the water is being frequently changed: indeed, in order to economise the silver solution, this need not be begun until the second month, when the water is changed every fortnight.
As has been stated above, it is only necessary to read off the number of cubic centimetres of the solution used in the titration, for the decrease in these figures is a sufficient indication of the progress of the operation, while the diminution of the chlorine-content may be taken as an indication of the simultaneous removal of the sulphates[88]. In the treatment of small objects in distilled water, the process may be regarded as complete if the red colour is obtained on the addition of from one to two drops (i.e. about 1/10 to 1/5 cubic centimetre) of the silver solution. If, when tap-water is used, and is being changed at intervals of a fortnight or a month, the estimations give a constant result between 0·6 and 1·0, the treatment need not be carried further.
