5. =HEAT.= Physiologic salt-solution is heated to 80°C. Thin pieces of tissue are placed in the hot water for two minutes, and then after-hardened in alcohol. For larger pieces of œdematous tissues, cysts, etc., that cannot be cut into thin pieces, the salt-solution should be brought to 100°C and the tissues boiled for several minutes. This method is advised particularly for the coagulation of albumin in cysts, œdematous tissues, for the study of renal casts, etc. 6. =MERCURIC CHLORIDE.= This is used most commonly in the form of =a concentrated water solution= (mercuric chloride 7.5 grms., sodium chloride 0.5 grm., glacial acetic acid 5 cc., water 100.0 cc.), or as =Zenker’s solution= (mercuric chloride 5.0 grms., sodium sulphate 1.0 grm., potassium bichromate 2.5 grms., water 100 cc.; dissolve by heating, add 5 cc. glacial acetic acid just before using. The use of a 5 per cent formol solution instead of acetic acid is recommended). The pieces of tissue should not be thicker than 5 mm. Fix 6-24 hours, then wash 24 hours in running water, and after-harden in alcohol. Should the sections show mercuric precipitates they should be treated with Lugol’s solution for 30-60 minutes, then washed in a dilute solution of lithium carbonate and thoroughly washed out in water and alcohol. Much better stains can be obtained by this treatment of the sections with Lugol’s. The use of iodine in the alcohol during the process of after-hardening is not advisable because of the action of the iodine upon the albuminates of mercury. A 5 per cent solution of =sublamine= in distilled water has recently been recommended by _Klingmüller_ and _Veiel_. Fix 1-3 hours, wash and after-harden in alcohol. Precipitates are not formed, and good staining-results are obtained. =Advantages.= The mercuric chloride solutions preserve well the red blood-cells, mitotic figures and finer details of cell-structure, and permit the staining of bacteria and animal parasites in the sections. Certain especial staining methods (Mallory’s reticulum-stain, etc.) can be used only after mercuric chloride fixation, while many others (Heidenhain’s iron-hæmatoxylin, Biondi-Heidenhain triple stain, etc.) give best results after this fixation. For ordinary work the saturated mercuric chloride solution is preferable to Zenker’s, as the latter does not give good results with the commonly-used hæmatoxylin stains. =Disadvantages.= More troublesome and expensive; require thorough washing and subsequent removal of precipitates, and affect (Zenker’s particularly) certain stains.