the economic coefficient of different light sources are considered and sources of ultraviolet rays are indicated. The fifth chapter studies in detail the absorption of light, Beer’s law, and the doctrine of chromophoric groups; further, the theory of absorption is briefly developed (without mathematics, with only the results indicated), and in the sixth chapter the author turns properly to photochemistry. Here Sheppard analyzes the general case of photochemical equilibrium and kinetics, considers reversible, irreversible, and pseudo-reversible reactions and allotropic changes. In the seventh chapter individual special cases of photochemical reactions are analyzed, and in the last four chapters (VIII, IX, X, XI) the relations of matter to light and the appearance of light in reactions are discussed, and organic synthesis under the influence of light is set forth in detail. In the textbook the author tries to make use not only of the English literature, but also of the German; unfortunately, in the exposition there are at times annoying inaccuracies in the transmission of results, and too little attention is paid to the fundamental laws. In any case, Sheppard’s book is a valuable aid which, together with the books of Plotnikow and Weigert, can introduce the beginner to the complex and rapidly developing field of chemical transformations under the influence of light.

P. Lazarev.

Theodore Lyman. The Spectroscopy of the extreme ultra-violet. London, Bombay, Calcutta and Madras. 1914. (135 pages) (Monographs on Physics).

In his excellently clear and thoroughly written monograph, Lyman endeavors to illuminate in general outline the entire field of the doctrine of short ultraviolet rays; in the first part he gives general indications concerning the methods of investigating radiation in the ultraviolet region (photography, the photoelectric cell, thermal effects), and then gives general indications of the results obtained for the absorption of solid and vapor-like bodies. In the second—the larger—part (pp. 29–133), Lyman gives a detailed description of the methods developed by Schumann and by himself; the description of the methods is accompanied by valuable technical instructions which make it possible to reproduce the apparatus in practice. Subsequently Lyman presents the results of observations of absorption in the extreme ultraviolet spectrum, observations of the emission of gases and solid bodies in the same region, and of photoelectric effects. The book is supplied with a valuable list of all Schumann’s works and with a detailed bibliography of the subject.

P. Lazarev.