Characters. - In colourless cubical crystals, with no odour, but a pungent saline taste.

Preparation. - Vide p. 605.

Solubility. - It is readily soluble in water, less soluble in spirit.

Reactions. - Its aqueous solution gives the reactions of potassium (p. 603) and a bromide (p. 594).

Impurities. - Iodide and bromate.

Test. - For the iodide, vide p. 560. Bromate is detected by adding dilute sulphuric acid to the crushed crystals. They should not at once assume a yellow colour. The acid liberates hydrobromic acid from the bromide, and if bromate be present the reaction between it and the hydrobromic acid liberates free bromine. 5HBr + HBrO3 = 3H2O + 3Br2.

Action. - Bromide of potassium does not seem to have, like the iodide, any marked influence on the lymphatic system, and although it has been occasionally used instead of the iodide in lymphatic swellings and enlargements of organs, this use of it is not general. When swallowed in small doses it produces no effect, but when taken in large doses for a considerable time it causes an eruption like acne upon the face, the complexion at the same time becoming muddy or bronzed. The chief symptoms are, however, impairment of the functions of the spinal cord and the brain. There is a great diminution of reflex action, so that touching the pharynx no longer produces any tendency to vomit, even though the touch itself be felt. There is drowsiness and heaviness, a great inclination to sleep and insensibility to outward impressions, the memory is impaired, the speech becomes hesitating and articulation imperfect, the intellect is less clear, the genital functions are much diminished, the gait becomes tottering and unsteady, and the muscles weak. To these symptoms the name of bromism is given.

Uses. - Its chief use is in nervous diseases for the purpose of producing sleep, allaying excitement, and diminishing spasm.

Bromide of potassium is most useful as a hypnotic in cases of sleeplessness due to mental excitement and worry. Some persons, after hard study or close attention to business, instead of sleeping at night are no sooner in bed than the brain seems to become doubly active, the carotids throb and they toss about from side to side trying in vain to get rid of the ideas which come in a constant train before them. In such cases when bromide of potassium is taken, the throbbing of the carotids and temporals and the fulness in the head disappear and sleep is induced. A dose of 10-15 grains given before bed-time may be sufficient in mild cases, but when the agitation is great 30 or 40 grains must be given, and should be assisted by cold ablution to the head and a prolonged warm foot-bath. The dose may be repeated, if necessary, every hour or two hours, until the desired effect has been obtained. One great advantage that bromide of potassium possesses over other hypnotics is that it can be pushed without fear, and the same is true of other bromides. They are not dangerous to life, and even when they are pushed so far as to cause bromism, the symptoms usually pass off rapidly when the drug is discontinued.

It is very useful in lessening the excitability, susceptibility to worry, and irritability of temper from which gouty persons often suffer. It should be given with a considerable proportion of water.

In delirium tremens where there is sleeplessness with fearful visions it may be given in doses from 20-30 grains or even more every two hours till sleep is induced. It is of most benefit in the earlier stages before the delirium has become furious, and is useful also at the end of the attack in dispelling delusions which may still remain.

During the latter months of pregnancy, women are sometimes troubled at night with the imagination that they have committed or are about to commit some great crime, such as murdering their husbands or children; and these delusions, according to Ringer, are removed by potassium bromide.

It is also useful, he says, in the treatment of night screaming in children, apparently allied to nightmare. They awake out of sleep screaming, seem very much frightened, and do not appear to recognise their mother or other friends who try in vain to soothe them. In the sleeplessness of mania it is frequently though not always successful. It may be used in fevers and inflammation when sleep is absent, and whenever opium and belladonna or hyoscyamus fail to produce sleep or cause sickness.

In convulsive nervous affections, such as whooping cough, laryngismus stridulus and spasmodic asthma, it is very useful, and also to some extent in St. Vitus's dance and hysteria.

It is especially beneficial in epilepsy, and by its use the convulsions can almost always be lessened if not entirely stopped. A similar result has been obtained in experiments on animals (p. 187). It is not so useful when the convulsions are violent, and it is not so beneficial when there is only a transitory loss of consciousness, as in petit mal. It is, perhaps, however, not so much a curative as an alleviative remedy, and the fits are apt to return when its administration is discontinued.

It is useful in relieving sickness, especially in pregnancy. In sea-sickness it is perhaps more useful than any other remedy. It should be taken in thirty-grain doses twice or thrice a day, for a day or two before the voyage begins, and should be kept up while it continues. In severe cases it may be necessary to push the bromide so far as to keep up a state of more or less somnolency and stupidity during the whole voyage.

From its power of lessening the sexual passion it is used as an anaphrodisiac in priapism and nymphomania.

It is also useful in menorrhagia, especially when this occurs in young women, according to Ringer, while Garrod says it is more useful in old women.

It is useful in neuralgia occurring in debilitated subjects, and sometimes accompanied by flushed face with cold hands and feet. It has been used in diabetes.