~Test Tubes~, from 5-1/4 to 5-1/2 inches long, and of such a diameter that they will hold from 20 to 22 cubic centimetres of water when filled to a height of 5 inches; rather thick glass is preferable. Indiarubber stoppers, fitting the test tubes, and carrying an arrangement for holding the test papers, viz., a narrow glass tube passing through the centre of the stopper, and terminating in a platinum wire hook. A glass rod drawn out and the end turned up to form a hook is better. ~The Thermometer~ should have a range from 30 deg. to 212 deg. F., or from 1 deg. to 100 deg. C. A minute clock is useful. ~Test Paper.~--The test paper is prepared as follows:--45 grains (2.9 grms.) of white maize starch (corn flour), previously washed with cold water, are added to 8-1/2 oz. of water. The mixture is stirred, heated to boiling, and kept gently boiling for ten minutes; 15 grains (1 grm.) of pure potassium iodide (previously recrystallised from alcohol, absolutely necessary) are dissolved in 8-1/2 oz. of distilled water. The two solutions are thoroughly mixed and allowed to get cold. Strips or sheets of white English filter paper, previously washed with water and re-dried, are dipped into the solution thus prepared, and allowed to remain in it for not less than ten seconds; they are then allowed to drain and dry in a place free from laboratory fumes and dust. The upper and lower margins of the strips or sheets are cut off, and the paper is preserved in well- stoppered or corked bottles, and in the dark. The dimensions of the pieces of test paper used are about 4/10 inch by 8/10 inch (10 mm. by 20 mm.).[A] [Footnote A: When the paper is freshly prepared, and as long as it remains in good condition, a drop of diluted acetic acid put on the paper with a glass rod produces no coloration. In process of time it will become brownish, when treated with the acid, especially if it has been exposed to sunlight. It is then not fit for use.] In Germany zinc-iodide starch paper is used, which is considered to be more sensitive than potassium iodide. ~Standard Tint Paper.~--A solution of caramel in water is made of such concentration that when diluted one hundred times (10 c.c. made up to 1 litre) the tint of this diluted solution equals the tint produced by the Nessler test in 100 c.c. water containing .000075 grm. of ammonia, or .00023505 grm. AmCl. With this caramel solution lines are drawn on strips of white filter paper (previously well washed with distilled water, to remove traces of bleaching matter, and dried) by means of a quill pen. When the marks thus produced are dry, the paper is cut into pieces of the same size as the test paper previously described, in such a way that each piece has a brown line across it near the middle of its length, and only such strips are preserved in which the brown line has a breadth varying from 1\2 mm. to 1 mm. (1/50 of an inch to 1/25 of an inch).