The evaporation of water is accompanied by the absorption of heat, which, in the case of spontaneous evaporation, results in a lowering of temperature. Since, at the same relative humidity, the evaporation of water is slower as the water to be evaporated is colder, it is seen that drying tends to become progressively slower. This retardation is all the more marked when evaporation takes place simultaneously from both sides of a negative ; a drop of water adhering to the back of a negative often results in retarding the drying of the portion of the gelatine laver immediately opposite to it.
424. Apparatus for Drying. Glass negatives are generally dried in the vertical position 1 in the grooves of a draining rack. These accessories are not always made in the most rational form, folding racks being as a rule the worst offenders. In the first place they are often unsteady, being so designed as to cause the diagonal of the plate to be very far from vertical, which is the most stable position. Secondly, the grooves are almost always too close together, allowing an insufficient circulation of air between the plates —so much so that it is no uncommon thing to find water which has been evaporated from one coating condensed as dew on the back of the next plate. This is generally avoided by using only one groove in every two or three. If care has been taken to wipe the glass sides of nega tives before placing them in the racks, two negatives may be placed back to back in adjoin ing grooves, each pair of negatives being separated from the next by at least one empty groove. Another plan is to arrange the nega tives all facing the same way, in order to avoid mistakes ; a constant interval is then left between each.
If, for any reason, a plate has to be dried without a rack, it may be rested against the wall with its lower edge on several thicknesses of clean white blotting paper.
Cut films, if they have been developed, fixed, and washed in developing hangers, are generally kept in these hangers until drying has been carried out, the hangers being suspended from special racks or placed in ordinary plate racks.
In the absence of hangers, films may be sus pended from a stretched string in the open air, or inside a drying cabinet by hooks made from pins bent in the form of the letter or they may he pinned to the edges of shelves. Whatever
method be adopted, films must be far enough from each other and from neighbouring objects to prevent their sticking together or to some other object, should they wave to and fro in a draught.
Lengths of film which are not much more than 3 ft. or 4 ft. long are generally hung by special clips to rods or stretched wires, the lower ends of the films being weighted by other clips of sufficient size.
Long lengths of film (cinematograph films, aerial film negatives, etc.) are dried by winding them on special skeleton drums, which are kept uniformly turning on their axes, or causing them to pass along a series of vertical pipes in which a current of moderately warmed air circulates in the opposite direction.
425. The Operation of Drying. Drying should be carried out in premises which are well venti lated, dry, and at a moderate temperature. Freedom from dust is another requirement, and for this reason it should be in a room where there is no need for constant passing to and fro—a certain means of raising dust. In commercial installations drying is carried out in special rooms into which filtered air, dry and warm, is forced.
It is essential that during drying there should be no sudden change of temperature, of relative humidity, or of speed of air currents. Every negative which is subjected to great changes in conditions of drying shows a distinct mark be tween the portions dried under different condi tions. Those parts which have been dried most slowly are sometimes denser and sometimes less dense than others. In particular, negatives must not be allowed to stand in the sun during drying.
Before being set to dry, negatives should he wiped on the gelatine side (on both sides in the case of films) either with a rubber sponge which has been well squeezed out in water or with a grease-free chamois leather which has been soaked and wrung out.' For wiping a very small number of negatives, they may be placed between two sheets of fluffiess blotting paper or between the leaves of a book of filter paper, th hand being then passed over the paper with moderate pressure. In this way the duration of drying is considerably reduced, and also drops of water arc prevented from causing marks due to the local retardation of drying.