Chloral (trichloraldehyde) is a linpid, colourless, oily liquid, with a peculiar penetrating odour, and is formed when anhydrous alcohol is acted on by dry chlorine gas. It dissolves sulphur, phosphorus, bromine, and iodine, and is closely allied to Aldehyde (q.v.). Chloral combined with one equivalent of water forms chloral hydrate, a white crystalline substance, with a pungent odour and a bitter taste, to which the name chloral is commonly though incorrectly applied. Chloral was discovered by Liebig in 1831, and investigated by Dumas; the chloral hydrate was first used as an anaesthetic and hypnotic by Liebreich in 1869. The chief action of a moderate dose of chloral (15 to 30 grains) is the production of sleep, closely resembling natural sleep, and usually sound and refreshing. It has also a marked effect in quieting excitement, as in insanity or delirium; and in relaxing spasm, and checking convulsions and allied conditions. Its action as an anaesthetic is very capricious and uncertain; medicinal doses sometimes relieve pain completely, but much more often fail to do so. It lessens the force of the heart's action, and in large doses greatly reduces the temperature of the body; and to these effects the fatal results that sometimes follow its administration are chiefly due. As a hypnotic it is most valuable in cases where opium or morphia is dangerous or undesirable (in children, in disease of the kidneys), and where sleeplessness is combined with excitement (delirium of fevers, delirium tremens, insanity); but it may be employed in many other cases with advantage. In tetanus (lockjaw), and other diseases attended by convulsions, it is often of great value. It acts as an antidote in poisoning by strychnia and Calabar bean. It must be employed with the greatest caution, or not at all where there is any reason to suspect weakness of the heart, or embarrassment of the circulation from any other cause; in such cases dangerous symptoms are very readily produced by it. When habitually employed to procure sleep, it is generally less hurtful than opium; but sometimes 'profound melancholy and enfeeblement of the will, muscular lassitude, inability to sleep without the drug,' and other untoward symptoms (called collectively chloralism) result, and only disappear when its use is discontinued. Moreover, fatal accidents from its indiscriminate use are far from uncommon. Poisoning by chloral should be treated by keeping the patient warm, attempting to rouse him, administering coffee and small doses of strychnia or atropia.
When chloral hydrate is treated with caustic potash, pure chloroform is obtained; but owing to the expense, this process has not come into use in Great Britain. It has been supposed that its anæsthetic property is the result of a similar formation of chloroform in the blood, but no evidence of this is forthcoming.