The Dictionary Of Photography

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

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Fahrenheit                                                                Feer-type
quent decomposition may give rise to the formation of some compound of sulphur and silver. Again, insufficient fixing may-form the insoluble variety of hyposulphite of silver, and in-sufficient elimination of the fixing salt itself may supply the unlooked-for result. Hypo is used as an anti-chlor in the manufacture of some mounts, and this should be tested for as described under the head of Sodium Hyposulphite (q.v.). An acid or decomposing mountant, or absorption by an hygroscopic mount of the aqueous vapour from the atmosphere holding in solution certain acids, may also cause it. To prevent any such action, great care should be taken to completely eliminate the fixing salts, and to keep the prints where damp is not likely to reach them; the use of encaustic paste is in itself a partial pro-tection from the aqueous vapour.
Fahrenheit. See Thermometer.
Falling Front. See Rising Front.
Feer-type. This process is founded on the property possessed by diazo compounds (see Diazotype) of forming diazo-sulphonic salts with sodium sulphite ; and these diazo-sulphonic compounds, when mixed with phenols or amines and exposed to light, are decomposed, and the diazo compound is set free and forms a colouring matter - a positive is thus obtained from a negative. This process was patented by Dr. Adolf Feer in 1889, and the following is the substance of his specification : - The present process rests on the fact that, as the inventor has discovered, diazo-sulphonic salts react with phenol alkalies and hydrochloric or free aromatic amines, under the influence of solar or electric light, with the formation of the azo colouring matter. To carry out this process the inventor impregnates paper or cloth with a diluted mixture of a diazo-sulphonic salt {e.g., of aniline, amido azo-benzol, benzidine, and their homologues) and alkaline com-pounds of phenols (as phenol, resorcin, a- and) on one hand, or hydrochloric and amines (aniline, naphthylamine, phenylendiamine, and their homologues) on the other hand ; the paper or material is then dried in the dark, and exposed under a negative for about five minutes, or to the electric light. By this means there is formed on the exposed places the insoluble azo colouring matter, whilst on the unexposed parts, under the opaque portions of the negative, the preparations remain in their