The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Formic Acid
if this be suspected the light should be tested as described under D ark-Room.
Forceps. Vulcanite or horn forceps are very useful in printing operations, as they can be washed freely. In mounting prints the small flat-ended spring forceps used by stamp collectors are serviceable in dealing with irregularities and in lifting corners for relaying. The steel should be protected as indicated under Clips.
Formalin. The liquid sold under this name is a solution in water of the gaseous aldehyde of methylic alcohol; this aldehyde having the composition CH20, and the commercial solution usually contains 40 per cent. The solution of formic aldehyde has such tendency to harden gelatine and make it insoluble, that a film, whether on glass or on paper, can be made to resist the action of even boiling water by a soaking for five minutes in a mixture of 1 part of the commercial formalin with from 8 to 12 parts of water. It is seldom, however, that so strong a solu-tion need be used, 1 part to 20 or 25 of water being sufficient for the prevention of frilling if ten or fifteen minutes is allowed ; moreover, the strong solution evolves irritating vapours, some-times causes opalescence of the film, and occasionally brings about a condition of extreme brittleness or even a falling into dust. If negatives are soaked for ten minutes in the weaker formalin solution as given above, after the final washing, the film may be surface dried with blotting paper, and then completely dried in a few minutes before a fire. A lantern slide similarly treated may be exhibited at once in the lantern without fear of the film melting. One disadvantage of using formalin is that subsequent intensification or reduction becomes very difficult. Some workers rinse the negative, transparency, or print on gelatine emulsion paper, in a few changes of water after fixing, then soak for about five minutes in a 10 per cent, formalin bath, and after this wash in boiling water. Formalin is a powerful antiseptic - one drop to a quart serving to preserve even milk from change.
Formic Acid. (Ger., Ameisensaure ; Fr., Acide Formique). HCHOa=46. An acid liquid prepared by oxidation from various organic materials, but was originally obtained from ants, whence