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of amœbæ, and the opening directions for obtaining them will give a good idea of the clearness, directness, and simplicity with which the whole work is written:

Amœbæ are frequently to be found in abundance in the superficial ooze which forms a thin layer upon the bottom of nearly every quiet body of fresh water. The ooze may be collected from a pond, stream, or ditch, by gently and slowly skimming the bottom with a tin dipper fastened to a long handle. In gathering the ooze be careful to barely skim the surface, and to avoid disturbing the black mud which usually occurs just below the ooze.

Transfer the material thus gathered to a collecting-bottle, and gather ooze from several bodies of water, preserving each specimen in a separate bottle, for amœbæ may be abundant in one locality and almost absent in another. Pour the ooze into shallow dishes, such as soup plates or baking-dishes, putting enough into each dish to form a layer about an eighth of an inch deep over the bottom.

Place the dishes near the window, where they will be well lighted without exposure to the direct rays of the sun; fill them with fresh water, and allow them to stand undisturbed for two or three days, in order to allow the amœbæ to creep out of the ooze and accumulate at its surface.

If a permanent supply of amœbæ is desired, each dish may be converted into a small aquarium by the addition of a few floating water plants, such as "duck-weed," and when covered with a pane of glass, to exclude dust and prevent excessive evaporation, may be kept in good order for several months by simply replacing with fresh water the loss by evaporation.

In a day or two a thin brownish-yellow film will usually be visible over the whole or parts of the surface of the ooze; and portions of this film, almost entirely made up of microscopic organisms which have crept to the surface, may now be examined for amœbæ, in the following manner.

It only remains to add that this work is published at a very low price. Considering its cost and elegance, we hardly know of another so cheap a book. And, considering that there is no other book at all like it to serve the purposes of introductory study in its field, it ought to be in wide demand by the students of natural history.

Essays on the Floating Matter of the Air, in Relation to Putrefaction and Infection. With Illustrations. By John Tyndall, F.R.S., M.D. D. Appleton & Co. Pp. 338. Price, $1.50.

Some of the researches contained in this book have appeared in the pages of "The Popular Science Monthly," and nothing, therefore, need be said to our readers that is merely commendatory of their interest. The volume is the result of extended researches into one of the obscurest of subjects—the nature, conditions, and influence of the invisible microscopic life of the atmosphere. Any inquiry into the dust and floating contents of the air, if thoroughly pursued, leads to the more subtile question of infinitesimal forms of life and their germs as floating elements of the breathing medium. Profound problems are here encountered: Are these germs spontaneously originated, or are they subject to the laws of propagation which govern all other grades of life? Again, are these germs the seeds of disease which affect the higher forms of life, and thus become of the highest moment to the physician and the hygienist? The import of the subject has been disclosed only within the last few years, and depends upon the perfection of the microscope, and the most refined researches into the nature and effects of fermentation and putrefaction. Many able men in different countries have been working, with intense assiduity, over different branches of this momentous inquiry; but it was on many accounts fortunate that Professor Tyndall, about a dozen years ago, saw its importance, and brought all his resources to bear upon its systematic investigation. That he has thrown much more light upon the subject by his skillful and extensive experiments, and that he has made very important contributions to the establishment of the germ theory of disease, will not be questioned. But in still another respect it is fortunate that he identified himself with its elucidation. By his rare power of exposition, and his wonderful clearness of statement, he has done more, perhaps, than all other writers to impress the medical profession and the public both with the vital importance of the subject and the advance that has been made in the establishment of its fundamental principles. His present book embodies the main results of his original work, and, what is more, it presents them in so lucid and inviting a form that all classes of readers will be equally pleased and instructed by his views.

There is, perhaps, in the whole field of science no illustration more striking than is here afforded of the fruitful practical