ACIDUM CARBONICUM - CARBONIC ACID, CO2, = 44 (Not Officinal)

This gas occurs in the atmosphere in the proportion of 2 to 6 parts in 10,000; the air contained in the interstices of arable land has more, and in some grottoes and natural hollows, communicating probably with ancient volcanoes, carbonic acid accumulates, so as to exert toxic effects. This is the case in the well-known Grotto del Cane at Naples, the Upas valley of Java, and in many parts of Auvergne and Vivarais ("estouffis"). The gas is contained also in all water in varying quantity, certain sparkling waters having a proportion of more than half their volume. It occurs in all the liquids of the organism, principally in the blood, but also, in less quantity, in the urine: in the former, it exists combined with alkali, chiefly soda, and also in a free state; in the latter Morin found a proportion of 20 cub. cent. to the litre: this was increased under administration of carbonated water, also after walking exercise: it was greatly diminished by free drinking of ordinary water. It originates in the chemical phenomena of combustion and nutrition which are constantly taking place in the tissues, and it readily passes by osmosis through the animal membranes.

Preparation

By treating any carbonate - usually carbonate of lime - with dilute hydrochloric acid: the resulting gas is passed into water under pressure, and a solution is thus obtained.

Characters

A colorless inodorous gas of slightly sharp taste. It is soluble in its own volume of pure water at ordinary temperature and pressure - much more soluble under increased pressure and lowered temperature of the water. The solution gives an acid reaction, and is "sparkling" from rapid escape of gas. Carbonic acid is much more soluble in water containing phosphates than it is in pure water, and conversely, water containing the gas can dissolve and retain in solution, carbonates and phosphates of magnesia, lime, iron, etc., which pure water cannot. The sp. gr. of the gas is 1.526 (atmospheric air taken as 1). It is twenty-two times heavier than hydrogen.

Absorption And Elimination

Carbonic acid is easily absorbed by denuded surfaces, and by mucous and serous membranes. That it may be absorbed also through the unbroken skin is apparent from the systemic effects produced not only by carbonic acid baths in general, but by keeping separate limbs in an atmosphere of the gas while the respiratory organs are protected from it (Collard de Martigny). If taken in solution into the stomach, it is said to be absorbed, if the viscus be empty - while if it be full, the gas is rejected by eructation and per anum as flatus (Lehmann). Up to a certain amount, it may be absorbed through the lungs by the blood. In any normal condition, the blood is never saturated with the gas, but is always ready to receive more as it is freshly formed in the tissues. It circulates partly dissolved by the serum and partly combined with alkaline salts. It is eliminated almost entirely by the lungs and the skin, but in small proportion by the kidneys; also by the large bowel.

Physiological Action (External)

When carbonic acid gas, undiluted, is brought into contact with the skin, it causes some prickling and sense of warmth, with or without redness; this is said to be most marked about the perineum and scrotum - the latter contracts under its influence. To this effect succeeds a certain degree of anaesthesia (Rotureau) or analgesia, which, however, is not complete enough for operative purposes (Demarquay). In contact with mucous surfaces, or the exposed cutis vera, the effects are more marked, and more quickly produced. The oculo-nasal membrane is especially sensitive to a current of the gas, while the uterine membrane, and even wounded surfaces show the anaesthetic effect without much previous stimulation.

Physiological Action (Internal)