3. =CHROMIC ACID AND SALTS.= Chromic acid is rarely used alone in pathologic work, but is a constituent of Flemming’s solution (see below). Its salts are employed in the form of:— A. =Müller’s Fluid= (Potassium bichromate 25.0 grms., sodium sulphate 10.0 grms., water 1,000.0 cc.) This was formerly the favorite fixing solution, but is now used chiefly for the eye and nervous tissues, either alone, or after formol fixation, or mixed with formol. Large pieces may be used, even an entire brain, but the process requires months or even a year for the best results. Even small pieces take several weeks. The process may be hastened in the incubator. The solution should be changed whenever it becomes cloudy. When fixation is complete the fixed and hardened tissue may be cut directly on the freezing-microtome or after-hardened and dehydrated in alcohol or acetone when it is to be imbedded. The dehydration in alcohol should be carried out in the dark and without previous washing of the tissue in the case of nervous tissue. Greenish or brownish chrome precipitates appear if the dehydration takes place in the light; but for ordinary material this precipitate can be avoided by washing the fixed material in running water for 24 hours before transferring to alcohol. Moulds grow luxuriantly in Müller’s fluid, but may be inhibited by the use of pieces of camphor, thymol, etc. =Advantages.= Cheap, penetrates well, causes little shrinking, permits special nerve-stains, preserves red blood-cells, gives beautiful results with ordinary stains, preserves fat. =Disadvantages.= Slowness; does not preserve division-figures; does not permit of staining for bacteria; does not give good results with many special staining methods (fibrin, elastic tissue, reticulum, etc.). B. =Erlitzky’s Fluid= (Potassium bichromate 25.0 grms., copper sulphate 5.0 grms., water 1,000.0 cc.). Used for fixation of nervous tissue. The formation of pigment precipitates may lead to misinterpretation: the artefacts may be removed by hot water or dilute acetic acid. C. =Orth’s Fluid= (Müller’s fluid 100 cc., formol 10 cc. Make fresh before using, as the mixture precipitates on standing.). Fix for 3-12 hours in the incubator, or for 24-48 hours at room temperature. Wash in running water for 12-24 hours; cut on the freezing microtome, or after-harden in acetone or alcohol and imbed. =Advantages.= Combines the good features of Müller’s and formol fixations, and obviates some of the disadvantages. It is a good general fixing solution.