The fluid used must be one which will not itself injure the specimen, and which can be thoroughly removed by washing, so that it may not interfere with staining operations. The specimens should be kept while hardening in wide mouthed bottles, on the bottom of which a little cotton wool or tow has been laid. This allows the hardening fluid to come freely in contact with the under surface of the pieces of tissue, and prevents their being flattened against the hard glass bottom. The hardening fluid requires changing occasionally. This should always be done at the end of twenty-four hours, in order to get rid of any deposit of blood, &c., that may have accumulated. Besides this, the tissue when placed in the fluid contained a good deal of water which will have diluted it and consequently an early change is desirable. Afterwards the fluid requires to be changed only as often as it becomes turbid, or any deposit occurs, usually about once a week. While hardening, specimens should be kept in a cool place, as warmth favours changes in the cells, &c. In manipulating the portions of organs, forceps should always be used and these with great gentleness. The specimens should never be impaled with needles, or unsightly holes, which may even be mistaken for pathological appearances, will appear when a section is examined under the microscope. It requires some practice to know when the tissue is sufficiently hardened. The object aimed at is to make them not really hard but tough. It is almost unnecessary to add that in testing this with the fingers the utmost gentleness must be observed, or serious damage may be done to the tissue. When the tissue is sufficiently hardened the hardening fluid must be thoroughly dissolved out. This is most quickly effected by placing the specimen in a basin into which cold water from a tap is constantly running. The tissue may then be removed (forceps always being used and never the needle) and placed in an imbedding medium as subsequently directed; or, if it is not to be cut at once, into equal parts of methylated spirit and water, in which it may be kept indefinitely, the fluid being changed if it becomes at all cloudy. It is unnecessary for ordinary work to have more than the following hardening fluids:--*Müller’s fluid*:-- Potassium Bichromate 2-1/4 grms. 3-1/2 drachms. Sodium Sulphate 1 grm. 1-1/2 drachms. Water to 100 c.c. 1 pint. Two drachms of carbolic acid are sometimes added to each pint of the fluid but as a rule it is not necessary. Müller’s fluid is the most generally useful of the various fluids employed, for the following reasons:-- 1. It causes very little shrinking of the elements of the tissue, and hence may be employed for most delicate objects, _e.g._, the retina and embryos.