7. =OSMIC ACID.= Osmic acid is used alone in a 1 per cent solution, or in such combinations as =Flemming’s solution= (chromic acid 1 per cent. sol. 15 cc., 1 per cent osmic acid 4 cc., glacial acetic acid 1 cc.), =Hermann’s solution= (same as Flemming’s, with 15 cc. of a 1 per cent platinic chloride substituted for the chromic acid), =Altmann’s solution= (5 per cent potassium bichromate solution 50 cc., 2 per cent osmic acid solution 50 cc.), =Marchi’s solution= (Müller’s fluid 2 parts, 1 per cent osmic acid solution 1 part), and that of =Pianese= (1 per cent sodium-chloroplatinate 15 cc., 2 per cent osmic acid 5 cc., ¼ per cent chromic acid 5 cc., formic acid 1 drop). The pieces of tissue must be very thin, as osmic acid penetrates very slightly. Fix 6-24 hours in the dark, and wash thoroughly in running water, and after-harden in graded alcohols. These solutions have but limited use in pathology, and are used chiefly for the study of fat (oleates) and mitotic figures. Flemming’s and Hermann’s solutions are the best for the study of mitotic figures, the latter bringing out plasma details more clearly. Marchi’s fluid is used especially for the study of nerve-degeneration, and Altmann’s for the demonstration of Altmann’s granules. The method of Pianese is used for the demonstration of cell-inclusions. The osmic-acid mixtures are all expensive, penetrate poorly, cause precipitates, and affect greatly the staining-power of the tissues, so that it becomes necessary to use certain stains (safranin, carbol fuchsin, aniline gentian violet, etc.) as counterstains. 8. =PICRIC ACID.= A saturated water solution of picric acid is usually employed. Fix 12-24 hours, and wash in alcohol, not water, and after-harden in graded alcohols. Preserves mitotic figures, fine details of cell-structure, and is very good for bone and calcified tissues, as it decalcifies and fixes at the same time. Numerous modifications and combinations of the above methods have been proposed such as _Flemming’s chrom-acetic solution_, _Rawitz’s chromic-picric-nitric fluid_, _Rabl’s chrom-formic mixture_, _Burckhardt’s chrom-osmic-nitric solution_, _Merkel’s fluid (chromic acid-platinic chloride)_, _Carnoy’s mixture (glacial acetic acid 1_, _absolute alcohol 6_, _chloroform 3_), and many others. They have a limited use in pathologic work. In the judgment of a section as to its fixation the following points may be of service: in alcohol fixation the red blood cells are haemolyzed, and there is much shrinking; with formol fixation the red cells stain copper-red with eosin; in mercuric chloride fixations the red cells stain rose-red with eosin, and pigment precipitates are present; in bichromate fixation the red cells preserve their natural color, and fat cells show a brownish color; osmic-acid fixation is shown by the black color of the oleates, and the failure of the tissue to stain by ordinary stains.