1. =ACETONE.= A water-free acetone is employed by placing pure dried white copper sulphate in the bottom of the bottle or vessel in which the fixation-process is carried on. Several layers of filter-paper are put over the copper sulphate to keep the tissues from touching it, and the acetone is then poured into the vessel. As soon as the copper-sulphate becomes blue it must be again fused. It is only by this method of constant dehydration that acetone can be employed to any advantage as a fixing agent; if fused copper sulphate is not employed the amount of acetone necessary to fix well is so great that the method becomes too troublesome and expensive to be recommended. But with the simple copper-sulphate method of constant dehydration acetone becomes the cheapest, most rapid and one of the best fixing reagents. The use of alcohol is avoided, and the period of infiltration in xylol and in paraffin shortened. For very quick work the entire process of fixation, hardening and dehydration may be achieved by the use of acetone alone, small pieces of tissue being fixed ½-2 hours in acetone and then transferred directly to soft paraffin. For ordinary work pieces of tissue 0.5 cm. thick are put into acetone over fused copper sulphate for 20-60 minutes. A judgment of the degree of fixation can be obtained by pressing the tissue lightly between the fingers; if it is of uniform consistence, and does not give as if the inner portions were softer than the surface, the fixation is complete. From the acetone the tissues may be brought directly into xylol for 5-10 minutes, until they obtain a cloudy transparency, thence into paraffin for 15-30 minutes and then blocked. The whole process of fixation, imbedding, cutting and staining can be carried out in 30 minutes. Acetone may be combined with formol, alcohol or any of the other fixing agents, but when it is desired to use any one of these for some especial purpose it is better to fix first with the desired reagent and then to use acetone for the dehydration-process alone, instead of alcohol. We have found that formol-fixation followed by acetone dehydration gives excellent results for general pathologic work. =Formol-acetone= (acetone 100 cc., formol 10 cc.) may be found to have advantages. The quick fixation in water-free acetone causes less contraction than fixation with absolute alcohol; fixation with graded acetone-solutions causes practically none.