Chloral hydrate is a geminal diol with the formula C₂H₃Cl₃O₂. It is a colorless solid. It has limited use as a sedative and hypnotic pharmaceutical drug. It is also a useful laboratory chemical reagent and precursor. It is derived from chloral (trichloroacetaldehyde) by the addition of one equivalent of water. It was discovered in 1832 by Justus von Liebig in Gießen when a chlorination (halogenation) reaction was performed on ethanol. Its sedative properties were observed by Rudolf ://www.jstor.org/stable/44450759 Buchheim in 1861, and then described in detail and published by Oscar Liebreich in1869, and subsequently, because of its easy synthesis, its use was widespread. It was widely used for sedation in asylums and in general medical practice, and also became a popular drug of abuse in the late 19th century. One notable recreational user, for instance, was the poet and illustrator Dante Gabriel Rossetti. Chloral hydrate is soluble in both water and ethanol, readily forming concentrated solutions. A solution of chloral hydrate in ethanol called "knockout drops" was used to prepare a Mickey Finn. More reputable uses of chloral hydrate include its use as a clearing agent for chitin and fibers and as a key ingredient in Hoyer's mounting medium, which is used to prepare permanent or semipermanent microscope slides of small organisms, histological sections, and chromosome squashes. Because of its status as a regulated substance, chloral hydrate can be difficult to obtain. This has led to chloral hydrate being replaced by alternative reagents in microscopy procedures. It is, together with chloroform, a minor side-product of the chlorination of water when organic residues such as humic acids are present. It has been detected in drinking water at concentrations of up to 100 micrograms per litre (µg/L) but concentrations are normally found to be below 10 µg/L. Levels are generally found to be higher in surface water than in ground water. Chloral hydrate has not been approved by the FDA in the United States or the EMA in the European Union for any medical indication and is on the FDA list of unapproved drugs that are still prescribed by clinicians. Usage of the drug as a sedative or hypnotic may carry some risk given the lack of clinical trials. However, there are chloral hydrate products, licensed for short-term management of severe insomnia, available in the United Kingdom.