To assess the value of an antacid for medicinal purposes the obvious method is to study the effect on the gastric secretion in the stomach of the human subject. This subject may be a normal person or one with hyperacidity or, if emphasis is placed on clinical use, one suffering from an ulcerous condition. It is very difficult however to secure reliable information by this method. There are very wide variations in the constitution of the gastric juice of the same individual even within a short space of time and according to Mutchl critical assessment of the relative healing or prophylactic virtues of antacids is impossible.It may well be that the results of well designed chemical tests will supply more reliable information than in vivo experiments which might at the first sight appear to be more desirable. Such chemical tests, however, should approach as nearly as possible the normal conditions which might exist in the stomach of a patient who is taking an antacid to control hyperacidity. The acid absorption test for magnesium trisilicate of the British Pharmacopeia 1953 consists in adding to the antacid a considerable excess of 0.05N hydrochloric acid and stirring continuously at 20" C . for 4 hours and determining the amount of acid neutralised by titrating the excess with 0.05N alkali. The neutralising capacity test for aluminium hydroxide gel of the British Pharmaceutical Codex 1949, is similar but the reaction is carried out at 37" C. for one hour. These tests are of value for the purpose for which they were designed, i.e., to ensure that the medicinal substance has a certain minimum capacity to neutralise acid. No attempt is made to approach the conditions under which the antacid will be expected to act medicinally except that the British Pharmaceutical Codex directs that the test be carried out at about body temperature, 37" C. If the results of such tests are used as criteria for evaluating antacids it is possible that very erroneous conclusions may be reached. The conditions of these tests differ from those occurring in actual use in that there is (a) a considerable excess of acid at all times during the test and this may not be the case in the stomach, (b) the effect of buffers etc. present in the stomach is not allowed for, (c) in the stomach there is a continuous and variable secretion of acid and formation of other substances, and ( d ) there is continuous removal of the stomach contents to the duodenum.These factors must have a considerable effect upon the in vivo action of antacids and must be considered when attempts are made to correlate the results of chemical tests with the probable action in the stomach. Hammarlund and Rising2 added hydrochloric acid at intervals of 30 minutes to the antacid and recorded the p H every 5 minutes for 3 hours. Alstead3 used human gastric juice for his tests and Mutchl also added hydrochloric 692

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Summary 8 samples of tincture of digitalis have been compared chemically and biologically with a tincture prepared from the standard preparation of digitalis. The results of the frog assays do not agree with those of the chemical assays using the 3:5‐dinitrobenzoic acid reagent. A new method for the decolourisation of tinctures of digitalis has been investigated. The tincture is passed through an alumina column which adsorbs the pigments, and the eluate can be directly assayed with the Kedde reagent. The total time taken for decolourisation is at the most 5 minutes and the loss of colour‐producing active constituents—presumably due to adsorption on the alumina is less than 10 per cent. The constituents of 9 samples of tincture of digitalis have been identified by paper partition chromatography.