FORMALDEHYDE, the first member, H.CHO or of the series of saturated aliphatic aldehydes (q.v.) is most readily prepared by passing the vapour of methyl alcohol, mixed with air, over heated copper or platinum ; the formaldehyde vapour is condensed and absorbed, either in water or alcohol. At ordinary temperatures formaldehyde is a gas possessing a pungent smell; it is a strong antiseptic and disinfectant, a 4o% 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 hexamethylenetetramine, and "condenses" in the presence of some mineral bases to produce compounds which apparently belong to the sugars (q.v.). It renders glue, casein or gelatin insoluble in water, and is used in the coal-tar colour industry in the manufacture of pararosaniline, pyronines and rosamines. Several polymerides have been described. Para-formaldehyde, obtained by concentrating solu tions of formaldehyde in vacuo, is a white solid, which on heating changes back into the original form. By cooling gaseous formal dehyde a crystalline polymeride, trioxymethylene, is formed this also reverts to the simple monomeric form on heat ing. The strength of solutions of formaldehyde may be ascer tained by the addition of excess of standard ammonia to the alde hyde solution (hexamethylenetetramine being formed), the excess of ammonia being then estimated by titration with standard acid. Scientific interest attaches to the formation of formaldehyde by the oxidation of methane at high temperatures (W. A. Bone) and by the action of ozone on this hydrocarbon. Formaldehyde also appears to be a reduction product of carbon dioxide and the first stage in the assimilation of this oxide by the green plant during photosynthesis (see CARBOHYDRATES), whereupon poly merization of this aldehyde leads to the sugars and other carbo hydrates.