A fourth is osmic acid, used in 1 per cent. solution. This is a highly valuable reagent, but extremely expensive, very poisonous, and giving off fumes which are most irritating to the eyes. The fifth, a very gentle, but in many respects very satisfactory one, is picric acid in saturated solution. Tissues preserved in this medium must not be washed out with water, as it enters into very feeble combination with protoplasm, and the cells swell and disintegrate as the reagent is dissolved out. Of mounting media we may mention glycerine, glycerine jelly (made by dissolving starch in glycerine with the aid of heat), and Canada balsam, dissolved in xylol or benzole. The Canada balsam must be dried hard by evaporation over a water-bath, and dissolved as wanted. Under no circumstances should raw balsam be used, as it takes years to set hard, and turns of a deep yellow colour in the process. Chloroform is frequently used as a solvent, but it has the disadvantage of attacking and extracting a large number of the aniline dyes used for staining structures, an objection from which the mineral solvents are free. We will now proceed to go through the objects already referred to, and indicate the method of preservation. For the study of the cell-structures of plants the portion to be examined is to be placed in spirit of about 30 per cent. strength, which is changed after twenty-four hours for 40 per cent., after a further twenty-four hours for 55 per cent., and finally, as regards our present purpose, in 70 per cent. spirit, in which it may remain until required for section-cutting. The effect of this treatment is to extract the bulk of the water from the tissue, with the minimum of shrinkage of the cells, the latter being preserved in their natural relations to surrounding parts. In some cases, however, it is desirable to examine and preserve delicate structures, or parts, or dissections, in a medium which allows of the retention of the greater part of the natural moisture, and in such a case the tissue is immersed in glycerine diluted very much in the same way as the alcohol in the last process, but with very much longer intervals between the alterations of strength, until it reaches pure glycerine, in which it remains for a considerable time, as the exchange between the tissue and the dense fluid surrounding it goes on very slowly. A combination of the two methods is also possible, the spirit-hardening being carried out for a portion of the time, and the tissue being thereafter transferred to glycerine, diluted or pure.