FORMALIN, or FORMALDEHYDE, CH2O or H.CHO, the first member of the series of saturated aliphatic aldehydes. It is most readily prepared by passing the vapour of methyl alcohol, mixed with air, over heated copper or platinum. In order to collect the formaldehyde, the vapour is condensed and absorbed, either in water or alcohol. It may also be obtained, although only in small quantities, by the distillation of calcium formate. At ordinary temperatures formaldehyde is a gas possessing a pungent smell; it is a strong antiseptic and disinfectant, a 40% solution of the aldehyde in water or methyl alcohol, sold as _formalin_, being employed as a deodorant, fungicide and preservative. It is not possible to obtain the aldehyde in a pure condition, since it readily polymerizes. It is a strong reducing agent; it combines with ammonia to form _hexamethylene tetramine_, (CH2)6N4, and easily "condenses" in the presence of many bases to produce compounds which apparently belong to the sugars (q.v.). It renders glue or gelatin insoluble in water, and is used in the coal-tar colour industry in the manufacture of para-rosaniline, pyronines and rosamines. Several polymers have been described. _Para-formaldehyde_, or trioxymethylene, obtained by concentrating solutions of formaldehyde _in vacuo_, is a white crystalline solid, which sublimes at about 100 deg. C. and melts at a somewhat higher temperature, changing back into the original form. It is insoluble in cold water, alcohol and ether. A diformaldehyde is supposed to separate as white flakes when the vapour is passed into chloroform (Korber, _Pharm. Zeit._, 1904, xlix. p. 609); F. Auerbach and H. Barschall (_Chem. Zentr._, 1907, ii. p. 1734) obtained three polymers by acting with concentrated sulphuric acid on solutions of formaldehyde, and a fourth by heating one of the forms so obtained. The strength of solutions of formaldehyde may be ascertained by the addition of excess of standard ammonia to the aldehyde solution (hexamethylene tetramine being formed), the excess of ammonia being then estimated by titration with standard acid. On the formation of formaldehyde by the oxidation of methane at high temperatures, see W.A. Bone (_Journ. Chem. Soc._, 1902, 81, p. 535; 1903, 83, p. 1074). Formaldehyde also appears to be a reduction product of carbon dioxide (see _Annual Reports of the Chemical Society_).