Externally. - Acids are irritants, and some of them very powerful corrosives. The strong acids are used as caustics; nitric acid to destroy chancres; acetic acid, warts; sulphuric acid, some forms of malignant growths. Very dilute watery solutions, sponged on the skin in fever, cool the surface by evaporation, and thus act as refrigerants; whilst watery solutions of sulphuric acid used in this way appear to constringe the tissues, and diminish the sweating of phthisis.

Internally. - In the dilute form, acids act directly upon the contents of the alimentary canal, and are used in the treatment of poisoning by alkalies. In every instance the free acids quickly unite with bases in the digestive tract, and form neutral salts. In the mouth they are stimulants and sialagogues: they relieve thirst, rouse the appetite, and aid digestion by increasing the flow of saliva and gastric juice, the citrates, tartrates, and acetates being chiefly used for this purpose as acid drinks and fruits of great variety, e.g. in fever. In the stomach hydrochloric acid increases the acidity of the gastric juice, and is given for this purpose during or after meals, as a powerful stomachic. Carbonic acid, introduced in effervescing wines and waters, has a grateful sedative action upon the gastric nerves; and in the form of champagne and effervescing mixtures is a most valuable remedy in the treatment of sickness with exhaustion. The other acids assist gastric digestion but to a very small, possibly useless, extent. Reaching the duodenum, acids increase the acidity of the chyme and stimulate the liver, pancreas, and intestinal muscles and glands. Dilute nitric and nitrohydrochloric acids, given at the end of meals, are therefore used as cholagogues in intestinal dyspepsia with hepatic torpidity, especially in tropical cases.

2. Actions On The Blood And Their Uses

Acids render the blood less alkaline (but never acid, even in poisonous doses), by combining with part of the alkali of the liquor sanguinis. No special use is made of this property. Phosphoric acid increases the phosphates in the red corpuscles, and is thus haematinic. The vegetable acids, when given as salts of the alkalies, have an important deoxydising effect on the blood. For example, citrate of potash becomes converted in the blood into carbonate of potash, carbonic acid, and water, a portion, however, of the citric acid always remaining unoxydised (see Potassium), thus: 2(K3C6H5O7) + O18 (in blood) = 3(K2CO3) + 5H1O, + 9CO2. Citrates, tartrates, and acetates of potash, soda, ammonia, etc., in the effervescing form, may therefore be used to set free in the blood the carbonates of the alkalies, which cannot be so conveniently or safely given in large doses by the stomach. The vegetable acids have been used in the treatment of scurvy, apparently with doubtful success; and in rheumatism, with equally questionable results, beyond their action on the mouth, skin, and kidneys.

3. Specific Actions And Uses

In the tissues and organs each of the acids exhibits a specific action of its own. Sulphuric Acid is an astringent to the bowels, skin, and blood-vessels, and is a valuable remedy for diarrhoea, profuse sweating, and haemorrhage. Nitric and Nitrohydrochloric Acids are cholagogue, specifically as well as locally; e.g. when administered by means of a foot-bath (8 fl.oz. to one gallon of water), or of a compress wrung out of the solution and worn over the hepatic region. Tropical enlargements of the liver may thus be reduced. The tonic influence of these acids is probably referable to their stimulating effect upon the gastric and biliary functions. Hydrochloric Acid enters the tissues as chlorides, and no specific action or use can therefore be credited to the small doses which can be given of it. Phosphoric Acid also possesses no further influence on the tissues than that of increasing pro tanto the amount of phosphates, and possibly the growth of bones; and its value in constitutional diseases is probably due to its action on the red corpuscles, and to the bases with which it is combined (iron, lime, etc.). As we have seen, Acetic, Citric, and Tartaric Acids never reach the tissues, being decomposed in the blood, unless given in large doses.

4. Remote Local Actions And Uses

The acids, having chiefly entered into combination as neutral salts, or having been decomposed in the blood, produce remarkably little local action when they are escaping from the body in the secretions. Sulphuric acid is excreted chiefly by the kidneys, increasing very slightly the normal amount of sulphates; part escapes by the bowels as sulphates; part by the skin, this acid being anhidrotic. Photphoric and Hydrochloric acids behave similarly. Nitric acid is believed to be partly decomposed into ammonia, and thus actually to diminish, to a slight degree, the acidity of the urine. Acetic, Tartaric, and Citric acids pass out of the body as carbonates, unless in excess, when they escape unchanged by the kidneys. The important point to be noted about all these acids. therefore, is, that they do not, to any considerable or useful extent, increase the acidity of the urine. It must be observed, however, that all the acids probably stimulate the kidneys and skin indirectly, by increasing the total amount of salts excreted by them.