Characters. - Small, crimson, needle-shaped or columnar crystals, deliquescent, odourless, having a caustic effect upon the skin and other animal tissues, and an acid reaction. Very soluble in water, forming an orange-red solution. Brought in contact with alcohol, mutual decomposition takes place. When heated to about 190° C. (374° F.) chromic acid melts and at 250° C. (482° F.) it is mostly decomposed, with the formation of dark green chromic oxide and the evolution of oxygen. On contact, trituration, or warming with strong alcohol, glycerine, spirit of nitrous ether, or other easily oxidisable substances, it is liable to cause sudden combustion or explosion.

Tests. - If 1 grain of chromic acid be dissolved in 100 c.c. of cold water and mixed with 10 c.c. of hydrochloric acid, the further addition of 1 c.c. of test solution of chloride of barium should cause not more than a white turbidity (limit of sulphuric acid).

Officinal Preparation.

B.P

Liquor Acidi Chromici (acid 1, water 3 parts).

Action. - It has a great power of coagulating albumin, and destroying low organisms, and as it parts very readily with oxygen it oxidises organic matter and decomposes ammonia and sulphuretted hydrogen. It is thus a powerful deodoriser and disinfectant. It is chiefly used as a caustic to destroy condylomata, and morbid growths in the mouth, larynx, or uterus, and to phagedenic ulcers, poisoned wounds, etc. As a solution of 1 in 40, it has been especially recommended in syphilitic affections of the tongue, mouth, and throat. As a lotion, it has been employed to lessen foetid discharges, and as an injection in ozaena, leucor-rhoea and gonorrhoea. Care must be taken not to prescribe it with any substance to which it readily yields oxygen, such as alcohol, glycerine, etc, as the mixture may explode spontaneously.

1 Extra Pharmacopoeia, Martindale and Westcott.

Acidum Carbonicum. Carbonic Acid, C02; 44. Not officinal. It is very extensively used dissolved in water, as aerated water, effervescing soda, potash, or lithia waters, or in wine, as champagne.

Properties. - Colourless gas, heavier than air, causing a pungent feeling in the nostrils. Soluble in its own volume of water. Its solubility is increased by the presence of carbonates, or by pressure, and when this is removed the gas escapes and causes the fluid to effervesce. The solution has an acid reaction. Carbonates of magnesium, calcium, iron, etc, which are only sparingly soluble in water, are dissolved with comparative ease by water holding the gas in solution.

Action and Uses. - Like other acids, when applied to the skin it acts as an irritant, but only slightly. After a prolonged application it causes a slight reddening of the skin and a feeling of warmth, which changes on the continuance of the application into burning or prickling, felt most where the skin is thin and richly supplied with nerves, as the external genitals, and this is not unfrequently accompanied by sweating. Carbonic acid baths (p. 469) are therefore sometimes used in catarrh and rheumatism as a slight rubefacient to the whole skin, and to cause sweating, especially where they can be obtained with ease, as in places where there are springs containing much carbonic acid. These baths - e.g. the ferruginous carbonic acid baths of various continental spas - have an aphrodisiac action and may be useful in sterility.

Carbonic acid has been used as a stimulant to ulcers, either by directing a stream of gas directly upon them or by applying a poultice of yeast (Cataplasma Fermenti, B.P.), which in the process of fermentation causes a constant production of this gas.

Streams of carbonic acid have been applied to the eyes, ears, nose, vagina, and rectum in catarrhal inflammation or ulceration of these parts, in order to cause a slight hyperaemia of the parts and healing of the inflammation and to diminish pain, as it is supposed to act locally by diminishing the sensibility of the nerves of the part.

In the mouth carbonic acid, like other acids, acts as a stimulant to the secretion of saliva, and so water containing it quenches thirst better than pure water, and it is therefore often used in feverish states (p. 360).

In the stomach it causes that slight pain which we confound with hunger, and a pleasant feeling of warmth just as on the skin. Here too it most probably causes a slight hyperaemia, and increased secretion. The greatest part leaves the stomach as gaseous eructations, but a portion is absorbed and enters the blood. Its action is thus transient, and it produces no material change in the chemical composition either of the contents or walls of the stomach. It increases the rapidity of the absorption of water in the intestinal canal, as is shown by the fact that water containing carbonic acid is excreted by the kidneys much sooner after it has been drunk, than water without it. It relieves irritation in the stomach, and allays or stops vomiting or nausea and slight derangements of digestion. Carbonic acid is naturally present in the intestines, in greater quantities in the large than the small. The carbonic acid is partly that which passes from the blood into the intestine in interchange for the oxygen contained in the air we swallow, and is partly formed by processes of fermentation which take place in the chyme.

That part of the carbonic acid which, after introduction into the stomach, passes into the blood is excreted by the lungs. Injected into the blood through a vein, it is likewise excreted in the same way without causing an injury, unless it is injected in such a quantity that some remains as gas undissolved in the blood, and then it causes death mechanically, just like air, by hindering the passage of blood through the lungs.

Poisoning by Carbonic Acid. - When it is inhaled, the ordinary interchange between the carbonic acid in the blood and the oxygen of the air is prevented, the gas in the blood accumulates, and the processes of oxidation in tissues being interfered with, their functions are lessened or destroyed (p. 262).

The nervous system is first affected, and there is headache, beating or singing in the ears, giddiness, flushing of the face. Then there is a feeling of want of breath, tightness of the breast, palpitation of the heart and great anxiety. If the C02 be still inhaled, the pulse becomes slower, consciousness is lost, delirium or coma ensues, and death occurs with convulsions.

In poisoning by carbonic acid three stages may be distinguished, (1) dyspnoea; (2) convulsions; (3) paralysis.

During the first stage the carbonic acid appears to act as a stimulus to the nerve-centres in the medulla, and especially to the respiratory and vaso-motor centres. In the second stage it stimulates other motor centres (p. 237). In the third it paralyses them. In the first stage, that of dyspnoea, the respirations are both rapid and deep, the inspiratory as well as the expiratory movements being increased. Both the inhibitory and the accelerating centres for the heart are stimulated, but the irritation of the vagus-roots preponderates, and the heart is generally slow. The vaso-motor centre in the medulla is also stimulated, and the blood-pressure rises. Besides this the carbonic acid also stimulates either subsidiary centres in the spinal cord (pp. 285 and 286), or acts directly on the walls of the vessels themselves, causing them to contract (p. 282), for the blood-pressure rises during inhalation' of carbonic acid even when the spinal cord has been divided below the medulla. The vessels of the surface become dilated. This is ascribed by Frankel to stimulation of a dilating centre. During the second stage, that of convulsions, the respiration becomes more and more laboured, and the expiratory movements greater, until general convulsions occur. The blood-pressure rises still more, the heart becomes still slower, and the right ventricle more distended. In the third stage, that of paralysis, the inspiratory movements become more and more feeble, the intervals between them longer and longer, and finally they cease. The vaso-motor centre becoming exhausted the blood-pressure falls, and this fall is probably aided by the action of the carbonic acid on the muscular walls of the blood-vessels themselves (p. 282), as well as by weakness of the heart. The heart generally continues to beat for some minutes after respiration has completely ceased, and if artificial respiration be commenced before pulsation is entirely arrested, life may generally be saved. Indeed, this is the case even when the cardiac pulsations are quite imperceptible, and therefore in cases of death from asphyxia it is well to keep up artificial respiration if possible for an hour or even longer, notwithstanding the apparent hopelessness of the case. It should only be discontinued when a ligature tied moderately tightly causes no trace of congestion in the finger-tip after being on for ten minutes, and it ought to be supplemented by intermittent pressure on the cardiac region in order to stimulate the heart. These observations apply not only to poisoning by carbonic acid, but to poisoning by all drugs which produce death by asphyxia, and to death by drowning.

Post-mortem examination shows great venous congestion everywhere, the right side of the heart being distended with blood, the brain much congested, with exudation and even extravasation, and the blood extraordinarily dark.

Treatment. - In cases of poisoning by carbonic acid, as in miners or men who have been suffocated in wells or brewers' vats, the great object is to get the blood oxygenated as quickly as possible. Get the person into the fresh air, and if the respiratory movements have ceased, dash cold water on the face and chest to awaken them reflexly. If this does not do, have recourse to artificial respiration. The next thing is to see that the heart is beating. When the right ventricle is distended with blood it becomes paralysed, and if it does not begin to beat shortly after artificial respiration has been begun the jugular vein should be opened in order to relieve the dilatation. There are no valves between the heart and the jugular vein (at least of any importance), so the blood flows directly out and the distended ventricle is relieved. One must, of course, be careful to prevent the access of air into the vein.