Page:Popular Science Monthly Volume 26.djvu/140

130 light. These discourses, which constitute the present volume, were delivered in November, 1883. In the first, the emission and the undulation theories of light are stated, and the insufficiency of the former is shown. Explanations of the phenomena of interference and diffraction by means of the undulation theory follow, and, in the third and fourth lectures, double refraction, polarization, and the interference of polarized light are considered.

compact volume is, first of all, a text-book of general chemistry, in which the substances and processes used in medical and pharmaceutical chemistry have been employed in illustrating the principles of the science. "From other chemical textbooks," says the author in his preface, "it differs in three particulars: first, in the exclusion of matter relating to compounds, which at present are only of interest to the scientific chemist; secondly, in containing more or less of the chemistry of every substance recognized officially or in general practice as a remedial agent; thirdly, in the paragraphs being so cast that the volume may be used as a guide in studying the science experimentally." The book contains also directions and tables for qualitative and quantitative analyses, and in these departments, likewise, special prominence is given to substances used as drugs. Naturally, organic compounds receive more attention than in the ordinary text-book on general chemistry, and a large amount of information, valuable to the physician and the pharmacist, is distributed through the volume.

and telephone operators, line-men, and others connected in similar capacities with the applications of electricity, have generally had scanty opportunities for education, and seldom possess sufficient understanding of electrical science to enable them to attain important and lucrative positions. It is especially for this class that the present book is written, though it is equally available for the general reader who wishes to understand the construction and operation of the electrical appliances which he sees in use. The text is arranged in the form of question and answer, the answers varying from a few lines to a couple of pages in length, and is illustrated with 152 cuts. Frictional and voltaic electricity, thermoelectricity, and magnetism, are taken up in successive chapters, questions about dynamo-electric machines are then answered, and two chapters are devoted to methods of electrical measurements, and to the terms and units used. The next eight chapters are devoted to telegraphy, beginning with the principles as exemplified in different systems, and including such subjects as line-construction and the adjustment and care of telegraph-instruments. Electric lighting, electro-metallurgy, electric bells, electro-therapeutics, and the telephone, are dealt with in single chapters, and a number of minor applications receive brief mention. Tables showing the weight and electrical resistance of various qualities of wire and other tables are appended.

book contains the substance of a series of articles originally published in "The Sanitary Engineer," with some new matter. It is intended to present the general principles which should guide a person in judging of various systems of, and appliances for, ventilation. Dr. Billings insists on the inseparable connection between ventilation and heating, and lays down as his first axiom, which applies especially to the large cities in our Northern States, that "in this climate it is impossible to have at the same time good ventilation, sufficient heating, and cheapness." After some preliminary considerations of heat and gases, the author takes up methods of heating, patent systems of ventilation and heating, means for removing dust and for