To obviate the difficulties, and to give greater precision and delicacy to volumetrical assays, the instrument known as Mohr’s ALKALIMETER, or Mohr’s BURETTE, and which is figured in the margin, may be employed. By means of it the test-acid in the graduated tube (_a_) may be added to the alkaline solution in (_f_), in any quantity at a time, however minute, by merely pressing the handles of the clamp (_d_) with the thumb and finger. The terminal tube (_e_) has its lower orifice very small, and it is connected with the burette by means of a small piece of vulcanised india-rubber tube, on which the clamp (_d_) acts. (See _engr._) The inner cylindrical part of the arm (_b_) is lined with cork, to prevent injury to the glass burette, and to hold it the more firmly. Generally the alkali in the specimen examined may be in either the caustic or carbonated state, or it may consist of any mixture of caustic alkali, or carbonates; but it is absolutely necessary for accurate results, that it should be free from sulphides, sulphites, and hyposulphites, as sulphuric acid acts upon these substances as well as on carbonates. The presence of chlorides does not interfere with the accuracy of the assay, unless a higher degree of heat is employed than that necessary for the expulsion of the absorbed carbonic acid. The SODA-ASH of commerce generally contains all these substances besides common salt, sulphate of soda, and insoluble matter, which do not interfere. Rough samples of POT-ASHES and PEARL-ASH also generally contain some sulphides, though not a large quantity. Various plans have been proposed to avoid this source of error. The best is that of MM. Fresenius and Will, given above, in which the value of the carbonates is estimated by their yield of carbonic anhydride. The difference between an assay of a sample of the unprepared alkali and of another which has been treated as above, indicates the quantity of impurities contained in them under the forms just referred to. The presence of these substances in the commercial alkalies may be detected by the following tests:—— _Sulphides._ The addition of sulphuric acid causes the evolution of an odour like that of rotten eggs. The sample in solution yields a black precipitate with acetate of lead. But the most delicate test is the splendid violet-blue colour with nitro-prusside of sodium. _Sulphites and Hyposulphites._ A solution of the alkali, insufficient for saturation, being added to sulphuric acid tinged reddish yellow with bichromate of potash, occasions a greenish tinge (owing to the formation of oxide of chromium), when these are present. Hydrochloric acid added to a clear solution, after some time, causes a turbidity and odour of sulphurous anhydride. _Chlorides_ yield a copious curdy precipitate with nitrate of silver, soluble in ammonia, and reprecipitated by excess of nitric acid.