External

All these acids are powerful irritants when applied externally. The feeblest is citric. Its concentrated solution has no action on the sound skin, but is irritant to mucous membranes and abraded surfaces. Tartaric is stronger than citric acid; it will act upon the unabraded skin, and applied to a sore it produces pain, a sensation of burning, and considerable vascular dilatation. The remaining acids are very powerful irritants, therefore even very dilute solutions of them may product considerable redness and perhaps vesication, and when the solu tion is strong they are very energetic caustics; sulphuric and phosphoric acids, having a powerful affinity for water, are especially active. Sulphuric acid leaves the carbon untouched, therefore it blackens; nitric stains the skin a deep yellow owing to the formation of picric acid (trinitro-benzol), it does not redis-solve the albumin it precipitates, and it is consequently limited in its area of action; nitrohydrochloric is very powerful; hydrochloric is the least active of the mineral acids; glacial acetic acid is useful when a limited action is required. Ricord's paste is composed of sulphuric acid and willow charcoal; Michel's, of sulphuric acid and asbestos. All the stronger acids unite with and coagulate albumin; hence weak solutions, not strong enough to form a slough, which by its separation may cause bleeding, will, by coagulating the blood and so plugging the vessels, and by coagulating the albumin in the tissues and so constricting the vessels, act as astringents and haemostatics. Citric acid is added to tablets of corrosive mercuric chloride so that when these are dissolved in making solutions the antiseptic shall penetrate into the tissues. Tartaric acid is used for the same purpose. Diluted solutions of acids are cooling to the flushed skin of fever, therefore they are called refrigerants.

Internal

Mouth. - All acids have a peculiar taste, and give rise to a feeling of roughness about the teeth. As the saliva is alkaline they increase the amount secreted, consequently by keeping the mouth moist they allay thirst.

Stomach

It is believed that, if given during a meal, acids will check the flow of gastric juice, as that is an acid secretion. Nitric acid, however, interferes with the digestion of proteids, as it combines with them. When the amount of acid secreted by the gastric mucous membrane is deficient, acids taken, after a meal, when all that the stomach can secrete has been secreted, aid digestion.

Intestine

Acids quickly become converted into neutral salts, and are probably absorbed as such. Some, especially diluted sulphuric, preserve in the intestine their astringent action. They increase the amount of bile poured into the intestine, and are hence cholagogues; this is especially the case with nitric acid. Nitrohydrochloric acid is a still better cholagogue, as it also increases the amount of bile secreted.

Remote Effects

Acids may render the blood less alkaline, but never acid. They do this by combining with some of the alkali of the plasma. As high alkalinity of plasma and tissues favors metabolism, acids slightly diminish it. They also diminish the carbon dioxide in the blood. Phosphoric acid is believed to increase the amount of phosphates in the red blood-corpuscles. The administration of hydrochloric acid will increase the number of red corpuscles in chlorosis, but it does not alter the amount of haemoglobin. It is probable that in their passage through the liver they check the formation of urea. The reason for this belief is that all these acids, except citric, acetic, tartaric and lactic, are excreted in the urine - especially in flesh feeders - chiefly as ammoniacal salts. Nitric acid is stated to be excreted to a small extent as ammonia, and hence slightly to increase the alkalinity of the urine. Acetic, citric and tartaric acids are decomposed in the blood, alkaline carbonates being formed, and the alkalinity of the urine is increased. This has already been discussed (see p. 128). Lactic acid is either converted into an alkaline carbonate, or passed out as carbon dioxide in solution in the urine. Some acids depress respiration in animals, and the respiration is restored by alkalies.