The assay solution may be acidified with acetic, sulphuric, or hydrochloric acid before titrating with "hypo;" but it must be only faintly so. An excess of acid may be nearly neutralised with ammonia without interference, but excess of alkali is fatal. Bicarbonate of soda must not be used in excess; it is best to avoid it altogether. The assay solution should be titrated at once, as it weakens on standing; and the "hypo" solution should be standardised every two or three days, as its strength is not constant. _The standard solution of hyposulphite of soda_ is made by dissolving 25 grams of the salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to 1 litre. 100 c.c. are equivalent to 1.27 gram of iodine. This solution is standardised by weighing, in a small beaker, about half a gram of iodine, to which is added a crystal or two of potassium iodide and a few drops of water. When dissolved, the solution is diluted to 100 c.c., and titrated in the manner described. The starch solution is made in the manner described under the iodide copper assay. 5 c.c. are used for each titration. In determining the effects of variations in the condition of the assay a solution of iodine was used, which was equivalent in strength to the "hypo" solution. It was made by dissolving 12.7 grams of iodine with 25 grams of potassium iodide in a little water and diluting to 1 litre. 100 c.c. of this solution were found (at the time of the experiments) to be equivalent to 102.0 c.c. of the "hypo." ~Effect of Varying Temperature.~--The bulk of the solution was 100 c.c.; 20 c.c. of iodine were taken, and 5 c.c. of starch solution were added towards the end as indicator. These conditions are also those of the other experiments, except where otherwise stated. Iodine being volatile, it is to be expected that with hot solutions low results will be obtained. Temperature 15° 20° 40° 60° 80° "Hypo" required 20.4 c.c. 20.4 c.c. 20.1 c.c. 19.2 c.c. 15.5 c.c. These show that the temperature should not much exceed 20°. ~Effect of Exposure of the Iodine Solution.~--Twenty c.c. of the iodine were diluted to 100 c.c., and exposed for varying lengths of time in open beakers at the ordinary temperature, and then titrated. Time exposed -- 1 day 2 days 3 days "Hypo" required 20.4 c.c. 16.1 c.c. 13.6 c.c. 9.4 c.c. ~Effect of Varying Bulk.~--These experiments were carried out in the usual way, bulk only varying. Bulk 100.0 c.c. 200.0 c.c. 300.0 c.c. 500.0 c.c. "Hypo" required 20.4 " 20.4 " 20.4 " 20.4 " ~Effect of Varying Acid.~--These experiments were under the usual conditions, the bulk being 100 c.c. The results were-- Acetic acid -- 1.5 c.c. 30.0 c.c. "Hypo" required 20.4 c.c. 20.7 " 20.7 " Hydrochloric acid -- 1.5 c.c. 15.0 c.c. "Hypo" required 20.4 c.c. 20.6 " 20.9 "