Carbonic acid water is prepared, according to the U. S. Pharmacopoeia, by forcing into water five times its bulk of carbonic acid gas. obtained by the reaction between marble and dilute sulphuric acid.

Water, under the ordinary atmospheric pressure, and at ordinary temperatures, absorbs about its own volume of carbonic acid gas, and may be made to take up any additional quantity that may be required by increasing the pressure; the quantity absorbed being directly proportionate to the augmentation of the pressure. Thus, if with the pressure of the atmosphere it will absorb its own volume, with a pressure double that of the air it will absorb two volumes, with triple the pressure three volumes, etc. As kept in the shops, the solution has not always the officinal strength; being sometimes stronger and sometimes weaker. I am told that the preparation generally kept by the druggists of Philadelphia contains about seven or eight times its bulk of the acid gas. This statement, however, applies only to the water first drawn from the fountains; as, being driven out by the pressure of its own escaped air, it gradually become weaker as the fountain is exhausted, and towards the close must be much more so than at first. The colder it is, the more gas it is capable of containing. To maintain its proper strength, it must be kept under steady pressure; and hence the best plan is to have it bottled by the manufacturer; in which case, it retains an equable strength for an indefinite length of time, if well secured.

Properties and Impurities. Carbonic acid water is an effervescing, sparkling, colourless liquid, of an acidulous, pungent, agreeable taste; often producing, when swallowed, considerable irritation in the fauces. It reddens litmus paper, and precipitates lime-water.

From the mode in which it is prepared and kept, it is liable to two impurities, which very much interfere with its beneficial operation. Not (infrequently the fountains or reservoirs containing it are furnished with a leaden tube of exit, so that a portion of the water, drawn at any one time, must have been for a longer or shorter period in contact with lead, and is liable to be more or less impregnated with the carbonate of that metal. Should it be frequently drawn, there will be no appreciable quantity of the salt of lead; but if not, the impregnation may be considerable. The water which has remained over night in the exit pipe often contains it; and I have known of two instances of colica pictonum. induced by the habit of drinking every morning the first draught of the water from the fountain.

Another impregnation is copper, derived from the fountain, which is usually made of that metal. The copper fountains are, it is true, tinned in the interior; but the process is not always well performed, or the tin coating is worn off in spots; and thus it very frequently happens that more or less copper is dissolved. Sometimes the solution is so strong as to be obvious to the taste.

It is highly important, for medical purposes, that the water should be quite free from these impurities. Their presence may easily be detected. If lead is contained in the preparation, it will be evinced by the production of a dark discoloration on the addition of hydrosulphate of ammonia; if copper, by a brown precipitate with ferrocyanide of potassium. This latter test is so delicate, that one part of copper, dissolved in 56,000 parts of the water, may be detected by the reddish tinge it produces.

Medical Effects and Uses

Carbonic acid is at first locally irritant This is perceived on attempting to inspire the pure gas, when a strong irritant impression is felt in the throat and air-passages, so strong, indeed, that the glottis closes spasmodically, and refuses to admit it unless diluted. Applied to the skin for a short time, it produces a feeling of warmth and tingling or prickling, which is said to be sometimes positively painful. When the strongly impregnated liquid is swallowed, it is often so irritant to the fauces, as with some persons to render it almost impossible to take a large draught of it without interruption. It exercises a similar excitant influence on the stomach itself, and this is probably one cause of its medicinal effects. In other words, it is a gentle gastric stimulant, operating in a manner more analogous to that of the milder aromatics than of any other medicines. It is said to be refrigerant, and to excite perspiration and diuresis. But I believe it owes these effects to the water with which it is taken; and one of its advantages is that, by the gentle stimulation of the acid, it prevents injurious effects from the large draughts of very cold water swallowed with it. I do not think that of itself it is stimulant to the secretions. It is probably not absorbed into the circulation as carbonic acid from the alimentary canal, for the tendencies of the blood are everywhere to give out rather than to take in that substance. Its first stimulant impression appears to be followed, as is known to be the case with the gas locally applied, by a sedative influence on the nervous tissue.

The gently stimulant action of carbonic acid water renders it useful as a tonic in dyspepsia and other states of gastric debility, if not used so largely and so frequently, as on the one hand to produce inflammation of the stomach, and on the other to exhaust its excitability. The dyspeptic patient will find advantage in taking a moderate draught of it twice a day. Another great advantage is the one, already referred to, of obviating too great a depressing effect from cold water, and of rendering it acceptable to the stomach, when it might otherwise prove oppressive, possibly excite gastric spasm, or be rejected. Hence, when heated and perspiring, we may much more safely take a draught of cold carbonic acid water, than an equal amount of equally cold water not similarly protected. Hence, too, in febrile diseases, carbonic acid water very cold may be given happily as a refreshing and refrigerating drink, when cold water in the same quantity might oppress the stomach. It thus, too, enables the liquid to act as a diluent, and, by entrance into the circulation, and passing off with the secretions, to relieve febrile excitement But the most useful remedial effect of carbonic acid is the relief of nausea and vomiting. There are few means more efficient for this purpose, when the nausea is not dependent on positive inflammation of the stomach. It is one of our very best resources in the irritable stomach of fevers. Most frequently, perhaps, the effect of carbonic acid, under these circumstances, is obtained through the medium of the effervescing draught; because there is usually, at the same time, an indication for the diaphoretic and refrigerant influence of the citrate of potassa. But the carbonic acid water is also much used, and is very efficient. The best method of employing it is to have a number of small bottles, each containing about two fluidounces, filled with the liquid, then well closed, and kept in ice-cold water. The contents of one of these may be taken, as wanted, every half hour, every hour, or less frequently; and the preparation remains unimpaired; whereas, if the liquid is used in successive draughts from one large bottle, the strength of it becomes exhausted before the bottle is emptied. It is not only the nausea of febrile disease that may be thus treated; but that also of cholera morbus, cholera infantum, and all other disorders in which the affection is properly gastric, and not positively inflammatory.

As a vehicle for laxative, diuretic, and diaphoretic medicines, in order to obviate any nauseating effect from them, and to render them acceptable to the stomach, carbonic acid water is much used. It is an excellent menstruum for Sulphate of magnesia, citrate of magnesia, or any other of the more soluble saline cathartics, of the citrates of potassa and ammonia as diaphoretics, and of the alkaline carbonates as diuretics and antilithies. It is also sometimes useful as a solvent for substances not soluble in water alone. The carbonate of iron and carbonate of magnesia are soluble to a certain extent in carbonic acid water, which thus offers the means of agreeably administering these substances in solution.

The dose of the preparation is not precise. The patient may, in general, be left to his own discretion; but it is best on the whole to give it in small quantities, as of a wineglassful, repeated frequently, than very largely at once. Seldom more than from four to eight fluidounces should be taken at one draught. It is often advisable to flavour it with some agreeable syrup; as, in febrile cases, with lemon syrup, and in dyspepsia with ginger syrup; but in nausea and vomiting, it is usually more effective when taken alone.

Considerable use has been made of carbonic acid water topically, as a gently stimulant agent. Applied by means of cloths, it has been employed advantageously in cancerous, sloughing, and other ill-conditioned ulcers, of which it relieves the pain, improves the secretions, and cheeks the gangrenous tendency. Fermenting poultices have been used for the same purpose. They owe their efficacy to the carbonic acid evolved.

Many natural mineral waters contain carbonic acid gas, which adds greatly to their usefulness, by rendering them more palatable and more acceptable to the stomach.