_Indicators._--The tincture of litmus used to show when the solution is exactly neutral is called an indicator, and many materials are used in a similar way in different analytical processes. Thus the indigo solution in Löwenthal's process is an indicator. A more useful indicator than litmus for tannery purposes is Dr. Lunge's "methyl orange," which is indifferent to carbonic acid, and may therefore be used in the cold with solutions of alkaline carbonates; which are much more easily made and preserved than those of the caustic alkalies necessary with litmus. It is very sensitive to mineral acids, but not equally so to organic. It may be obtained of Messrs. Mawson and Swan, of Newcastle; and as a minute quantity only must be used for each test, it is really cheaper than litmus, and a few _grm._ will last a lifetime. It must be dissolved in water, and not more than 2 or 3 drops taken for each titration. (Titration signifies an estimation by means of a standard solution.) Other indicators will be named in connection with the analytical methods in which they are used. [Illustration: Fig. 9.] _Instruments._--To practically carry out analysis by standard solutions, measuring glasses are required. One or more flasks marked in the neck to hold exact quantities (Fig. 9), one at least, holding 1 _litre_, are indispensable. One or two graduated cylinders (Fig. 10), holding 100 _c.c._, and divided into tenths of _c.c._, are very useful, and it is well also to have one holding a _litre_, and provided with a stopper (Fig. 11). This is called a "test mixer," but is not absolutely essential. [Illustration: Fig. 10.] [Illustration: Fig. 11.] [Illustration: Fig. 12.] [Illustration: Fig. 13.] [Illustration: Fig. 14.] Pipettes (Fig. 12) are tubes with a mark on the stem by which exact quantities of liquid can be taken. Several holding 5, 10, 20, and 25 _c.c._ are necessary, and one holding 10 _c.c._ and divided into tenths is advisable. Most important of all is the burette (Fig. 13). If only one is to be had, it must be a Mohr's burette with a glass tap, but as alkaline solutions are apt to set glass taps fast, it is well to have one with a tap, and another with a pinchcock (Fig. 14). They should hold 50 or 25 _c.c._ and be divided into tenths. The burette in use is fixed in a stand (Fig. 15) and filled up to the top of the graduation, and the quantity of solution delivered is then shown by the scale. It is usual to read by the under side of the hollow of the liquid, keeping the eye carefully level with it. [Illustration: Fig. 15.]