*Formal.*--An aqueous solution containing about 35 per cent. of formaldehyde. It is a rapid hardening agent, causes very little shrinkage of the tissues, and does not discharge the colour of the specimens as much as alcohol. For hardening formal should be used as a two to five per cent. solution in distilled water. It may also be used as a ten per cent. solution for mounting museum preparations, but there is some tendency for a cloudy deposit to form on the glass after a time. It is the most suitable hardening agent at our disposal for eyes. It rapidly fixes the tissue elements, but does not cause much contraction. It may also be used for hardening the brain and spinal cord. Large quantities of fluid must be used for the latter purpose and it must be frequently changed. As soon as they are sufficiently hardened they should be transferred to methylated spirit. *Marchi’s fluid.*--This consists of:-- Müller’s fluid 2 parts. Osmic acid solution (one per cent.) 1 part. It is used for hardening specimens as a preliminary to Golgi’s method for staining nerve cells (p. 97), and also to complete the hardening of sections of spinal cord, &c., before employing Schäfer’s modification of the Weigert Pal hæmatoxyline method (p. 91). It is also used as a stain for recently degenerated nerve tracts and fibres, especially after experimental lesions. The fluid has little penetrating power, and therefore tissues must be cut into small pieces, about 3/8 inch cube. It is not necessary to place them in this fluid at once on removal from the body, but the preliminary hardening must be in Müller’s fluid and not in alcohol, &c. SPECIAL HARDENING REAGENTS FOR RAPID FIXATION IN ORDER TO STUDY CELL STRUCTURE. 1. *Alcohol.* 2. *Flemming’s solution* (modified by Friedmann):-- Osmic acid (one per cent.) 3 c.c. (♏xxx.). Glacial acetic acid 2 c.c. (♏xx.). Chromic acid (one per cent.) 42 c.c. (℥j.) Small pieces should be hardened in this fluid for twelve to twenty-four hours, and then washed and transferred to alcohol for some days before staining. 3. *Nitric acid.*--A ten per cent. solution in distilled water. It hardens the tissue in three to four hours, and should be followed by 70 per cent. alcohol, the hardening being completed in absolute alcohol. In using any of these methods it is necessary that the tissue be removed from the body during life or immediately after death. They are employed for revealing the changes in the cells and their nuclei in rapidly growing or inflamed tissue, for studying karyokinesis in cancer cells, and investigating the appearance of nerve cells and gland cells at rest, when actively employed and when fatigued; and they are also most useful in preparing specimens of the “parasitic bodies” which have been described in many cancer cells. DECALCIFYING FLUIDS. Used in the preparation of bone, tooth, osseous tumours, &c. The two best fluids for general use are:--