Definition. - An aqueous solution containing not less than 37 per cent. by weight of absolute formaldehyde, H.COH.

Properties. - Formaldehyde itself is a gas at ordinary temperatures, having a very pungent odor. The various products on the market are solutions of the gas in water. They are variously known as formalin, formol, methylaldehyde, oxymethylene, methanal, etc Formaldehyde readily undergoes polymerization, whereby a solid form is obtained, known as paraformaldehyde, or paraform. When a solution of formaldehyde is evaporated by heat, and more slowly by long standing, paraformaldehyde separates as a white, flocculent, nearly odorless mass, which is almost insoluble in water, alcohol, or ether, and which begins to sublime below 100 C. When heated, paraformaldehyde vaporizes and reforms the gaseous formaldehyde. It is sold in tablets which are employed for disinfecting purposes by vaporization.

Formaldehyde is very active chemically; it has a strong reducing action and unites with ammonia, forming the official odorless hexamethylenamine (urotropine). It is easily oxidized.

Several dusting-powders containing formaldehyde in combination have been introduced; thus glutol is a compound of gelatin and formaldehyde, amyloform, of starch and formaldehyde, etc.

Formaldehyde is a constituent of many food-preservatives, embalming preparations, etc.

Therapeutics. - Formaldehyde solutions are efficient and valuable bactericides. They are used very extensively in mouthwashes, etc. The vapor and liquid are very irritating and are not well adapted to internal medication.