Fixatives.—A useful procedure is to fix the wet film by exposure to 4 per cent. osmic acid vapour for ten to thirty seconds, then place in absolute alcohol for five minutes to harden. Grade down from absolute alcohol through 90 per cent., 70 per cent., 50 per cent., and 30 per cent. alcohols to water. Stain wet with a suitable stain such as hæmatoxylin, and gradually dehydrate by grading through the necessary strengths of alcohol, clear in xylol or other oily clearing medium and mount in Canada balsam.
Other fixatives may be employed, such as are also useful for fixing pieces of tissue for sectioning. Films or smears on cover-slips while still wet are floated on the surface of the fixative in a watch glass. Some good fixatives of wide application are:—
Schaudinn’s Fluid.—This consists of a mixture of
| Saturated aqueous solution of corrosive sublimate | 2 | volumes |
| Absolute alcohol | 1 | volume |
Two modifications of Schaudinn’s formula may be found useful. A saturated solution of corrosive sublimate in physiological salt solution may be substituted for the aqueous one, and the addition of a few drops of glacial acetic acid to either of the preceding mixtures may be made.
Some workers prefer to use hot fixatives, raised to a temperature of about 50° C.
Fixation by corrosive sublimate solutions must be followed by thorough removal of the mercury salt by washing repeatedly in 30 per cent. alcohol or with iodine-alcohol.
Bouin’s Fluid, or modifications thereof, is also very useful for wet fixation. Bouin’s picro-formol solution consists of:—
| Saturated aqueous solution of picric acid | 30 | volumes |
| Formalin, 40 per cent. | 10 | " |
| Acetic acid, glacial | 2 | " |