Formic aldehyde --> formaldehyde
(Science: chemical) Commonly used fixative and antibacterial agent. As a fixative it is cheap and tends to cause less denaturation of proteins than does glutaraldehyde, particularly if used in a well buffered solution (buffered formalin, formal saline).
old formaldehyde solutions usually contain cross linking contaminants and it is therefore often preferable to used a formaldehyde generating agent such as paraformaldehyde. Formalin fumes, particularly in conjunction with hydrochloric acid vapour, are potently carcinogenic.