Before mounting in balsam, the specimen must be completely freed from water by means of absolute alcohol. (Sometimes care must be taken to bring it gradually into the alcohol to avoid collapsing.[16]) If an aniline stain has been used, it will not do to let it stay more than a minute or so in the alcohol, as the latter quickly extracts the stain. After dehydrating, the specimen should be placed on a clean slide in a drop of clove oil (bergamot or origanum oil is equally good), which renders it perfectly transparent, when a drop of balsam should be dropped upon it, and a perfectly clean cover glass placed over the preparation. The chloroform in which the balsam is dissolved will soon evaporate, leaving the object embedded in a transparent film of balsam between the slide and cover glass. No further treatment is necessary. For the finer details of nuclear division or similar studies, balsam mounts are usually preferable. [16] For gradual dehydrating, the specimens may be placed successively in 30 per cent, 50 per cent, 70 per cent, 90 per cent, and absolute alcohol. It is sometimes found necessary in sectioning very small and delicate organs to embed them in some firm substance which will permit sectioning, but these processes are too difficult and complicated to be described here. * * * * * The following books of reference may be recommended. This list is, of course, not exhaustive, but includes those works which will probably be of most value to the general student. 1. GOEBEL. Outlines of Morphology and Classification. 2. SACHS. Physiology of Plants. 3. DE BARY. Comparative Anatomy of Ferns and Phanerogams. 4. DE BARY. Morphology and Biology of Fungi, Mycetozoa, and Bacteria. These four works are translations from the German, and take the place of Sachs's Text-book of Botany, a very admirable work published first about twenty years ago, and now somewhat antiquated. Together they constitute a fairly exhaustive treatise on general botany.--New York, McMillan & Co. 5. GRAY. Structural Botany.--New York, Ivison & Co. 6. GOODALE. Physiological Botany.--New York, Ivison & Co. These two books cover somewhat the same ground as 1 and 2, but are much less exhaustive. 5. STRASBURGER. Das Botanische Practicum.--Jena. Where the student reads German, the original is to be preferred, as it is much more complete than the translations, which are made from an abridgment of the original work. This book and the next (7 and 8) are laboratory manuals, and are largely devoted to methods of work. 7. ARTHUR, BARNES, and COULTER. Plant Dissection.--Holt & Co., New York. 8. WHITMAN. Methods in Microscopic Anatomy and Embryology.--Casino & Co., Boston. For identifying plants the following books may be mentioned:-- Green algæ (exclusive of desmids, but including _Cyanophyceæ_ and _Volvocineæ_). WOLLE. Fresh-water Algæ of the United States.--Bethlehem, Penn.