It may be as well to state at the outset that physiological and pathological tissues can be hardened by immersion in methylated spirit alone, or a saturated solution of picric acid in methylated spirit in about a week, and it is said to yield satisfactory results, even some of the tissues being ready in twenty-four hours. The only drawback is that sections thus quickly hardened must be stained with picro-carmine. But, whatever method of hardening adopted, the tissue should be washed by means of a stream of water for half an hour, to remove all traces of the hardening agent, and on its removal pressed between folds of cotton cloth or fine Swedish filtering paper. The principal hardening re-agents usually kept in bulk ready for use are the following:-- _Absolute Alcohol._--This is suitable for the internal organs of animals, glands, &c. These organs must be perfectly fresh, and should be cut into small pieces, so that the alcohol may penetrate them as quickly as possible. The hardening is usually complete in a short time.[45] _Chromic Acid and Spirit._--Chromic acid one-sixth per cent., water solution two parts, and methylated spirit one part. This reagent hardens in about ten days. Then transfer to methylated spirit, which should be changed every day until all colour is discharged from the tissue. This is a suitable reagent for the preparation of cartilage, nerve trunks, heart, lips, blood vessels, trachea, lungs, tongue, intestines, and gullet. _Potassium Bichromate._---Make a two per cent. water solution of this salt. This will harden specimens in about three weeks. Then transfer the preparation to methylated spirit, and change it every day until all colour is discharged. This is suitable for spinal cord, medulla, cerebellum, and cerebrum. _Müller’s Fluid._--Bichromate of potash 30 grains, sulphate of soda 15 grains, distilled water 3-1/2 ounces. This hardens in from three to six weeks. Then transfer, as before, to methylated spirits, and change it every day until colour ceases to appear. Most suitable for lymphatic glands, eye-ball and its internal structures, as well as for tendons, and thymus gland. _Methylated Spirit_ may be generally employed, but it has a tendency to shrink some tissues too much; it hardens in about ten days. It is usual to change the spirit daily, for the first three days at least. Skin, mammary gland, supra-renal glands, tonsils, and all injected organs may be hardened in it. (See note on the adulteration of methylated spirit with rack-oil, which utterly spoils it for use.)