However spread, after the blood has dried, whenever it is desired to stain it, either at once or at any future and more convenient time, it should be "fixed" by dropping on the film a little absolute alcohol and ether in equal parts, or absolute alcohol alone. Cover- glass films may be dropped into a small wide-mouth bottle containing the fixing agent, or they may be fixed by passing them through the flame (not a good method), or by placing them for some hours (one to three) in a warm dry chamber at a temperature of 105° to 120° C. When alcohol is used, after ten to thirty minutes or longer it must be dried off before staining.
Stains
The stains usually employed belong to two categories: (a) those which stain the protoplasm and the nucleus the same colour; (b) those which, by tinting them differently, differentiate the protoplasm from the nucleus.
(a) Of the former I would recommend the following:—
Löffler's methylene blue.—Concentrated alcoholic solution of methylene blue 30 parts, solution of caustic potash (11000) 100 parts. Stain for thirty seconds, wash in water, dry and mount.
Carbol-thionin.—Saturated alcoholic (60 per cent.) solution of thionin 20 parts, watery solution (2 per cent.) of carbolic acid 100 parts. The mixture should be kept at least a fortnight before use. Stain for five minutes, wash and mount.
Hamatoxylin and eosin.—Ehrlich's acid hæmatoxylin, or strong solution of hæmalum, five minutes; weak eosin, half