34 FINGER PRINTS
CHAP.
The last preliminary to be noticed is the slowness with which the printer's ink 'hardens on the slab, and the rapidity with which it dries on paper. While serviceable for hours in the former case, in the latter it will be dry in a very few seconds. The drying or hardening of this oily ink has nothing whatever to do with the loss of moisture in the ordinary sense of the word, that is to say, of the loss of the contained water : it is wholly due to oxidisation of the oil. An extremely thin oxidised film soon forms on the surface of the layer on the slab, and this shields the lowerlying portions of the layer from the air, and retards further oxidisation. But paper is very unlike a polished slab ; it is a fine felt, full of minute interstices. When a printed period () is placed under the microscope it looks like a drop of tar in the middle of a clean bird's-nest. The ink is minutely divided among the interstices of the paper, and a large surface being thereby exposed to the air, it oxidises at once, while a print from the finger upon glass will not dry for two or three days. One effect of oxidisation is to give a granulated appearance to the ink on rollers which have been. allowed to get dirty. This granulation leaves clots on the slab which are fatal to good work whenever they are seen, the roller must be cleaned at once.
The best ink for finger printing is not the best for ordinary printing. It is important to a commercial printer that his ink should dry rapidly on the paper, and he does not want a particularly thin layer of it ; consequently, he prefers ink that contains various
