To mount pollen-grains, they should be sprinkled upon the surface of a slide which has been previously moistened with thin gum, and allowed to dry until it has become just “tacky”; the drying is then completed by gentle heat and a drop of balsam placed upon the grains, with a cover-glass over all. Bubbles will probably form, but with Canada balsam this is not of the slightest importance, as they always come out of their own accord, and balsam mounts should never be closed with cement of any kind until thoroughly dry. Air-bubbles in other media may be eliminated by the use of the air-pump shown in Fig. 16, which may be obtained from Baker at a very reasonable rate, and which is useful not only for that purpose, but for accelerating the drying of moist tissues. To do this, there is placed upon the plate of the pump a porcelain dish containing strong sulphuric acid, and upon this is placed a little triangle of platinum wire, which serves to support the preparation. The air is now exhausted; the tissue parts with moisture to supply its place, and this moisture is in turn greedily absorbed by the sulphuric acid, so that drying is rapid and continuous, as well as very thorough, whilst the process has the great advantage of dispensing entirely with the use of heat. [Illustration: Fig. 16.] Portions of many of the delicate algæ may be mounted in glycerine, having previously been soaked in it as already described; whilst the unicellular forms, such as desmids and diatoms, may be preserved in almost exactly the natural condition by simply mounting them in a saturated solution of picric acid. Probably formalin, in a solution of 10 per cent. strength, would answer the purpose equally well, but the writer has not tried it. It is hardly necessary to say that, with such extremely fluid media, great care is required in closing the cell. A thin layer of Hollis’s glue should be first painted on, to secure the cover in position, and when this is thoroughly dry, several successive layers must be added in the same way. It may be said here, that it is advisable in all cases to use circular cover-glasses, as far as possible, as they lend themselves with great facility to a mechanically accurate closure. This slide is placed upon a turn-table, carefully adjusted until the cover is seen to be central when rotated, and a brush, preferably a small camel-hair pencil, charged with the desired fluid, but not in large excess, is held against the junction of the slide and cover, whilst the table is rapidly spun. A little experience will teach better than any description what amount of fluid there should be in the brush, and how thick the cement should be. If too thick, it will drag off the cover; if too thin, it will flow over the latter and over the slide.