The first investigations upon this subject were conducted by Golding Bird in 1842, who summed up a series of most admirable researches by the statemerit that ' the matter brought up (i.e. vomited) contains considerable quantities of free hydrochloric acid during the more irritative stage of the disease, which gradually decreases in proportion to the decrease in strength, while the organic acids increase in proportion to the decrease of free hydrochloric acid.' The complicated method of analysis employed by Golding Bird was probably responsible in great measure for the neglect with which the subject was treated, for it was not until 1879, when v. den Velden published his observations upon the absence of hydrochloric acid in cancer of the pylorus, that the profession realised the important bearing of the discovery upon diagnosis. Immediately this essay appeared the subject was ardently taken up by Biegel, Ewald, Huebner, Honigmann, Thiersch, Jaworski and Gluczynski, Kahn and v. Mering, Bosenbach and others, who not only confirmed the general statements of v. den Yelden, but went so far as to declare that an absence of free hydrochloric acid constituted an almost infallible sign of carcinoma of the stomach. Like most other scientific conclusions that are formulated in haste, this one was duly repented of at leisure, for it was soon shown not only that certain cases are accompanied by the free acid throughout their entire course, but that other diseases occasionally exhibit a similar diminution of the acid secretion. Thus Ewald, Kahn and v. Mering, Steinon, Bosenheim, Bouveret, Waetzoldt, and other observers have recorded examples of gastric cancer in which the free acid not only persisted, but was present in excess ; while its total disappearance has been observed in atrophic gastritis, achylia gastrica, lardaceous degeneration of the stomach, certain infectious fevers (Wolfram), Addison's disease (Kohler), pernicious anaemia, Bright's disease, and many cases of phthisis (Bosenthal, Fenwick). We find that out of a total of 495 cases of gastric carcinoma, published by various authorities, where the contents of the stomach were carefully and systematically examined, free hydrochloric acid was absent in 89 per cent., and present in small quantities or at irregular intervals in 9.7 per cent., while in 1.3 per cent, it existed in excess. This conclusion tallies very closely with our own observations, which indicate that the colour tests for free hydrochloric acid give a negative reaction in 91 per cent, of all cases of the complaint.

Numerous tests have been proposed for the detection of free hydrochloric acid, but only two need be described.

(1) Dimethyl-amido-azo-benzol was introduced by Topfer on account of its delicacy and ease of application. In the form of a .5 per cent, solution in alcohol it possesses a golden-yellow colour, which instantly changes to cherry-red when brought into contact with the free acid. This reaction is readily observed by allowing a drop of the contents of the stomach to mingle with a small quantity of the solution upon a porcelain dish, when the characteristic colour is seen to develop at the junction of the two fluids.

(2) Phloroglucin and vanillin dissolved in alcohol was first recommended by Giinzberg as a convenient test for the presence of the free mineral acid, and has attained a world-wide popularity. In order to obtain the best results two grammes of phloroglucin and one gramme of vanillin are dissolved in 100 grammes of absolute alcohol, and the solution is preserved in a black well-stoppered bottle, as it is soon decomposed if exposed to the light (Boas). A few drops are mixed in a white china dish with an equal quantity of the filtered gastric contents, and gently warmed over a spirit lamp. If free hydrochloric acid is present, a beautiful rose colour will develop at the spot where the mixture dries. If too much heat is applied, a brownish-red tint is produced, even in the absence of the acid.

The diminished quantity of hydrochloric acid in carcinoma of the stomach was originally attributed by Riegel to its neutralisation by the alkaline secretions of the morbid growth, while according to Ewald the phenomenon was dependent upon failure of the general nutrition. These theories, however, were easily disproved, and it is now known that the failure of the secretion depends upon secondary inflammation and atrophy of the gastric glands. Thus Mathieu and Bosenheim have shown that in every case where free acid is absent during life the stomach presents signs of chronic inflammation, with a disappearance of the parietal cells and sometimes atrophy of the entire gland. This condition is a permanent one, since the acid does not reappear even after removal of the growth (Mintz).

These facts serve to explain the more uniform absence of free acid when the pylorus is involved than in those cases where the tumour is comparatively circumscribed and affects the walls or curvatures of the stomach, for it has already been shown that chronic gastritis is usually due to retention of the food and irritation of the gastric mucous membrane by the acid products of fermentation.

The continued existence of free acid along with carcinoma is chiefly met with under two conditions : (1) A small growth situated upon a wall of the stomach, (2) cancerous invasion of a simple chronic ulcer or its scar. In the former case the disease is more or less circumscribed, gastrectasis is slight, and, with the exception of that in the immediate vicinity of the tumour, the mucous membrane of the stomach is practically free from inflammation. In the latter condition hyper-secretion has usually existed for a considerable period of time, and the signs of it do not disappear until the morbid growth has given rise to a diffuse gastritis.

It is never sufficient to rely upon an absence of the acid in a single examination, but in every case the investigation should be repeated under similar conditions at least three times. This is especially important when the first attempt has been made without antecedent lavage or the administration of a test meal, for we have more than once noted a reappearance of the free acid after the stomach had been thoroughly cleansed for a few days and the patient fed by rectal injections. When these several precautions are adopted the continued absence of free hydrochloric acid constitutes valuable confirmatory evidence of the existence of a morbid growth.

The secretion of hydrochloric acid is seldom entirely abolished, since it can usually be detected in a combined state by means of appropriate tests; while according to Schule the determination of the total deficit is of much importance in the differential diagnosis of carcinoma and benign affections of the pylorus. Inasmuch, however, as the total amount of acid is often greatly reduced in atrophic gastritis and other diseases of the stomach, the value of the information obtained from a quantitative estimation is rarely commensurate with the labour expended upon it.