Twaddel's hydrometers are sold in sets of six or separately; they read us follows: -

No. 1. 0° to 24° = sp. gr. of l.00 to 1.12.

„ 2. 24° 480 = „ 1.12 „ 1.24.

„ 3. 48° „ 74° = „ 1.24 „ 1.37.

„ 4. 74° „ 102° = „ 1.37 „ 1.51.

„ 5. 102° „ 138° = „ 1.51 „ 1.69.

„ 6. 138° „ 170° = 1.69 „ 51.8.

The specific gravity of a liquid is determined by floating one of the hydrometers in some of the liquid, contained in a tall glass cylinder; if the hydrometer is suitable for this particular liquid, the instrument will sink until the surface of the liquid coincides with some mark on the stem of the hydrometer. Suppose the strength of a caustic soda solution is to be determined, and a No. 2 hydrometer is to be used, the level of the liquid reaching 30°, the gravity of the liquid is 30° Tw.; or, if multiplied by 5 and 1.000 be added, its true specific gravity, i.e. 115, will be obtained •, then the solution will be found to contain about 13 per cent, of caustic soda.