These are medicines which correct acidity in whatever part of the body they may encounter it. it must be recollected, however, that, in some parts of the system, the presence of a certain proportion of free acid is normal, and even essential to health. Thus, during the period of digestion, it is now universally admitted that the gastric juice is normally sour, and that this condition is greatly promotive of its solvent power; and a certain degree of acidity of the urine is required, in order that the phosphatic salts may at all times be held in solution. it is, therefore, only an excess of acid beyond the wants of the functions that requires correction. Such an excess frequently exists in the stomach and bowels. Though it is probably impossible that the blood should become positively acid during life, yet acid substances not unfrequently are either generated in it or find their way into it, and are rejected through the different emunctories, as the skin, lungs, and kidneys; and a sour breath, and sour odour of the perspiration, would evince such a state of system. Again, a considerable excess of acid is often met with in the urine, irritating the lining membrane of the urinary passages, and sometimes leading to the deposition of uric acid. This is not the place to treat of the causes of this excess, its symptoms, or its effects. These are subjects for the pathologist. Our business here is to point out the methods of correcting it; at least to indicate those measures by which the effects of acid, abnormally existing in the system, may be obviated, independently of any agency upon the functions.

1. Mode Of Operation

The most efficient method of accomplishing this object is by means of medicines which are capable of uniting with and neutralizing the acids; of converting these irritating substances into mild and innoxious neutral salts. Now any salifiable base will accomplish this object, so far as the acids in the alimentary canal are concerned; but it does not follow that they could all be used for this purpose with impunity. The oxides of silver, copper, antimony, etc. are capable of neutralizing acids, but in doing so they are converted into powerfully irritating, and even poisonous salts. it is only, therefore, those salifiable bases which form innoxious compounds with the acids they neutralize, that can be used. Of this kind are the medicinal alkalies and alkaline earths, and their carbonates, with the exception of baryta, which cannot be safely used for the purpose; and these are the substances employed as direct antacids.

But it is proper to make some discrimination between these antacids. Some are insoluble, or very nearly so, and consequently cannot enter the circulation until they have become neutralized. it is clear that these may be used for correcting acid in the primae viae, but not in the blood, or the secretions. They may, indeed, indirectly correct acid in these latter situations, by preventing its entrance into the circulation from the alimentary canal; but they cannot act upon that which may exist in the blood already, or may be generated in that fluid, or conveyed into it by the lymphatics from the disintegrating tissues. For this purpose the soluble antacids must be used, which, if in excess, after the neutralization of the acid in the stomach, may then enter the blood. These, after having restored the requisite degree of alkalinity to the blood, become themselves noxious, and are thrown off by the emunctories, and thus neutralize or alkalize the secretions. Sour breath and sour perspiration may thus be corrected, and the acidity of urine replaced by alkalinity of that liquid. Magnesia and its carbonate, and the carbonate of lime are in the former category; the alkalies and their carbonates, and lime-water, in the latter.

When the object is solely to alkalize the blood or the urine, it may be accomplished by the use of the neutral alkaline salts, of which the acid is of vegetable origin; as, for example, by citrate of potassa, tartrate of soda and potassa, and probably acetate of potassa. Experiment has shown that, under the use of these salts, in small doses frequently repeated, the urine becomes alkaline to test-paper. It is presumed that the vegetable acid is decomposed in the alimentary canal, and that the alkaline base combines with carbonic acid, and enters the circulation as a carbonate. All these salts, however, have been already sufficiently considered under other heads.

2. Therapeutic Application

In poisoning by the acids, as the sulphuric, nitric, muriatic, oxalic, and even tartaric in great excess, this class of remedies are the appropriate antidotes, and should always be promptly employed, along with free dilution, and evacuants.

Dyspepsia often offers strong indications for the use of antacids, to correct the cardialgia, gastric spasms, and other uneasiness so frequently attendant on that disorder. But it must be remembered that these medicines are here only palliatives; and that, in excess, they may do harm, partly by depriving the gastric juice of its normal proportion of acid, and partly by directly irritating the stomach. Their use, therefore, requires judgment, and they should not be given indiscriminately in dyspepsia, whenever any little uneasiness may seem, in the mind of the patient, to require them.

In infantile colics, and other digestive disorders, acid is a very frequent cause of the suffering, and the antacids are of great service, sometimes even relieving serious diseases, which appear to have their origin in irritation of the primae viae from this cause. In the colic of adults they will often also afford relief.

Diarrhoea is not unfrequently, both in adults and children, and especially in the latter, either produced or sustained by acid in the bowels, as indicated by the sour smelling and green passages, and sometimes also by sour breath and eructations. Hence antacids are among our habitual remedies in this affection.

Sick-headache, when dependent on an excess of acid in the stomach, may often be prevented or cured by a full dose of magnesia, or other medicine belonging to the class.

Febrile diseases, in their course, frequently evince, by the sour smell of the breath and the perspiration, the existence of acid in the blood, and call for the use of this class of medicines. This is especially apt to be the case in the fevers of children. It is obvious that those antacids vol. II.-55 should here be selected which are capable of being absorbed into the blood, as the alkaline carbonates or bicarbonates.

Gout and rheumatism often afford the same indication; and the alkaline method of treating them has strong advocates among those who believe that acid is the essential materies morbi in these diseases.

In diabetes there is, in many instances, a sourish as well as saccharine smell issuing from the patient, which proves the abnormal elaboration of acid in the system, calling for antacids; and, so far as medicines can affect the course of this fearful disease, the alkalies are among those most to be relied on.

The uric acid lithiasis offers another indication for antacids; and there are no means so efficacious in affording relief, in the gravelly and other urinary disorders dependent on an excess of uric acid or the urates in the urine. The soluble alkaline carbonates are, of course, here also specially indicated.

From the remarks which have been made, it is obvious that a considerable discrimination is necessary in the choice of the antacid, to adapt it to the circumstances of each case. Thus, in the bowel affections' magnesia and lime, or their carbonates, are usually preferable; the magnesian preparations being used when there is indication at the same time for a laxative effect, the calcareous, when it is desirable to produce rather an astringent effect on the bowels than otherwise. When the blood and urine are to be reached, the choice should fall on the alkalies or their carbonates; and here, as a general rule, the carbonates are to be preferred to the caustic alkalies, and the bicarbonates to the carbonates, as being milder in their operation on the mucous membrane of the primae viae.

Incidentally to their operation as antacids, these remedies often act beneficially as laxatives, refrigerants, diuretics, or diaphoretics, by means of the saline compounds which they form with the acids, and which may act on the bowels, or be absorbed.

But it must be remembered, in using the alkaline antacids for these various purposes, that all that is required is the neutralization of an abnormal excess of acid, and that it is desirable not to substitute an excess of alkalinity, unless some special indication for this condition exist.