In the examination of fresh tissue it is often desirable to use certain reagents for the purpose of making chemical tests or to bring out some structures more prominently than others. To introduce these reagents beneath the cover-glass in such a way as to get the desired effect without disturbing given fields requires some practice with a very simple technical method. The preparation is first examined in salt-solution, and the cover-glass adjusted so that it has a slight rim of fluid about its edge, but not enough to make it float. The reagent to be applied is dropped with a glass-dropper at one side of the cover-glass, while at the other the salt-solution is removed slowly by a piece of absorbent paper. The changes produced in the tissue-elements during the progress of the reagent can be observed under low or high powers. Care must be taken to change the fluids so slowly that isolated cells will not be washed away. 1. =Physiologic or Indifferent Fluids.= Serous exudates, blood-serum, hydrocele fluid, etc.; artificial serum made by a mixture of 9 parts physiologic salt-solution with 1 part white of egg; or physiologic salt-solution (0.9 per cent for warm-blooded animals, 0.6 per cent for cold-blooded). 2. =Maceration Fluids.= 33 per cent alcohol (24 hours); chromic acid 1:5000 (24 hours); potassium bichromate 0.1-0.2 per cent solution (2-4 days for nervous tissue); 0.1 per cent osmic acid (12-24 hours); 33 per cent potassium hydroxide (¼-1 hour, for muscle, tissue must be examined in the solution, as the cells dissolve when water is added); Arnold’s iodine solution (10 parts of a 10 per cent potassium iodide solution to which are added 5-10 drops of a solution containing 5 grms. of iodine and 10 grms. of potassium iodide in 100 cc. of water. Macerate one or more days. If solution becomes discolored add more of the second solution); very dilute formol solutions (1 cc. to 500 cc. physiologic salt-solution); Müller’s fluid (2-3 days, good for nervous tissue). 3. =Glycerin.= Used without diluting as a _clearing agent_, particularly when pigment is present; and as a _mounting medium_ for stained preparations that cannot be put into alcohol. 4. =Potassium Acetate.= Saturated water solution for clearing and mounting fresh preparations. Does not clear as strongly as glycerin, hence is better adapted for the examination of fresh tissues. 5. =Acetic Acid.= 1-2-5 per cent solutions are usually employed. Clears the protoplasm and causes the nucleus to shrink slightly and to stand out more distinctly. It differentiates fatty and albuminous granules, dissolving the latter; and is useful in the demonstration of elastic tissue fibres, sharply outlining these against the connective-tissue which swells and becomes clear.