Spirit is also frequently employed to complete the hardening by Müller’s fluid and to preserve tissues after they have been hardened.
About ten or fifteen times the bulk of spirit should be used for one of the tissues. The fluid should be changed on the third day and afterwards as required.
Müller’s fluid and spirit.—This is a useful combination for many purposes. It is made thus:—Müller’s fluid, three parts; methylated spirit, one part.
The fluid must be allowed to cool after mixing before being used, and if necessary filtered. It will harden specimens satisfactorily in three weeks.
Müller’s fluid and formal.—Is an extremely useful mixture made by adding one part of formal to nine of Müller’s fluid. It hardens in a much shorter time than Müller’s fluid and causes very little shrinkage.
Absolute alcohol.—Used as a hardening agent where the tissues are to be examined for micro-organisms, and for specimens to be stained by Nissl’s method (p. [101]). A cheaper and equally effective hardening medium is made by dehydrating methylated spirit by adding one ounce of fused carbonate of potassium to each pint of methylated spirit, and decanting.
Small pieces must be used. The depths of the block should not exceed 3/8 inch. The fluid should be changed on the third day. Hardening will be completed in about ten days or even earlier.
Osmic acid.—For rapidity of action, and for rapid fixing of all the tissue elements in their natural position osmic acid is one of the best hardening reagents we possess.
Its disadvantages as a hardening agent are:—
1. Its expense.