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THE POPULAR SCIENCE MONTHLY.

guishment in logic of questions of origin and validity. The difference between reality as understood in ordinary belief, and as the term is applied to science, is very definitely dealt with in the essay on What is Reality? also the query as to whether our feelings are more than our thoughts, and if space is actually occupied by the real. On the Phædo of Plato, the most interesting of the critical examinations apply to the distinction that ought to obtain between Plato's teachings as understood by himself, and as they are subsequently developed and interpreted by Aristotle. Comparing the arguments of the Phædrus with those of the Phædo, some technical points arise in the mind which Mr. Ritchie deems worthy of especial comment. Some striking objections to the position of Economics considered in its relation to the sciences are concisely recounted, and in Locke's Theory—property—the author suggests an interesting study on the theories of Hobbes and Locke in the light of events current in their day.

When the work is considered as a factor in modern research, each page and paragraph may be regarded as a brief historical and critical key to a few of the most striking questions engaging students of evolutionary philosophy.

Dictionary of the Active Principles of Plants; Alkaloids; Bitter Principles; Glucosides: Their Sources, Nature, and Chemical Characteristics, with Tabular Summary, Classification of Reactions, and Full Botanical and General Indexes. By Charles E. Sohn, F. I. C, F. C. S. London: Ballière, Tindall & Cox. Philadelphia: J. B. Lippincott Company, 1894. Pp. vii- 194. Price, $3.

The present work treats of nearly six hundred alkaloids, glucosides, and bitter principles, and it has been prepared in order that the details relating to these substances, now more or less scattered through chemical literature, should be so tabulated that not only a given attribute of any substance shall be readily found, but that there shall be information indicating wherein such a substance differs from, or resembles, another of its class.

The work is arranged in three parts: The first groups together the constituents of one plant or of a number of botanically or chemically allied plants, following as far as possible an alphabetical order. The second part consists of a tabular summary designed for ready reference as well as for contrasting one compound with another for analytical purposes. The third part is a classification of reactions for the special use of analysts.

There is a complete botanical as well as a general index to the volume. It is likely to prove a convenient work for the pharmacologist as well as the chemist.

 

A Treatise on Elementary Hydrostatics has been prepared by John A. Greaves (Macmillan & Co., New York, $1.10) with the purpose of treating the subject as fully as possible without using the calculus; but alternative proofs have been given where the calculus enables us either to obtain the results more easily or to express them more concisely. Having shown that solids may be classified according to their behavior under the action of forces, the author deduces the definition of a fluid from the characteristic behavior of all substances which we recognize as fluids. The special chapter headings are the Properties of Fluids, General Theories relating to Pressure, Center of Pressure, Floating Bodies, The Determination of Specific Gravity, Gases, Hydrostatic Machines, and Capillarity. In the last chapter it is shown from experiments that the energy of a material system depends partly on the extent of the surfaces separating the different substances. On the assumption of the existence of this surface energy, several well-known capillary phenomena are deduced.

For some time past it has seemed to G. A. T. Middleton that a concise work upon land surveying, in which modern instruments and modern methods of working were described, would be welcomed by many. The result has been the production of a small volume on Surveying and Surveying Instruments (Macmillan & Co., New York, $1.25), the substance of which has already appeared in a technical journal. It includes chapters on Surveys with Chain only. Obstructions in Chain-line and Right-angle Instruments, The Uses of the Level, Various Forms of Level and their Adjustments, The Uses of Anglemeasuring Instruments, The Theodolite and other Angle-measuring Instruments, and Instruments for ascertaining Distances.