532 



NA TURE 



[April io, 1902 



LECTURES ON ELECTRICITY AND LIGHT. 

 Leichifasslichc Vorlesuns^en iiber Elektrisitiit und Licht. 

 Hy Prof. Dr. G. Jaumann. Pp. xii + 375. (Leipzig : 

 Barth, 1902.) Price Mk. 6. 

 T N a book of 370 pages it is quite impossible that the 

 i subjects of magnetism, electricity and light could 

 be treated in anything but a scrappy manner, so that we 

 cannot expect from Prof. Jaumann anything more than 

 a general view of the subjects treated. The book under 

 review arose from a course of lectures at Prague for 

 beginners at the University and teachers in the secondary 

 schools, and is therefore of the nature of an outline 

 to be used as a guide in study supplemented by other 

 more technical reading. For this purpose, if there had 

 been a good selection of references to standard treatises, 

 the book would have been admirable ; but, unfortunately, 

 references are almost entirely absent. The author has 

 undoubtedly made a most interesting volume and has 

 treated the subject in a very original manner, dealing 

 with the phenomena of magnetism, electricity and light 

 from a physical point of view, using throughout the 

 Faraday conception of tubes of force. The first eighty- 

 four pages consist of an introduction dealing with 

 stationary stream lines in the motion of a liquid to lead 

 on to the conceptions of magnetic and electric lines of 

 force. The analogies between liquid stream lines due 

 to sources and sinks and vortices and lines offeree due 

 to charges and electric currents are well brought out, 

 and considerable ingenuity is exercised in constructing 

 cases of fluid motion to be analogous to the behaviour of 

 lines of electric force when more than one dielectric is 

 present in the field. It seems, however, curious to intro- 

 duce, for the benefit of readers who cannot be supposed 

 to understand lines of force, the lines of flow for a vortex 

 filament in a steady stream as the first case of stream 

 lines discussed. However, later on the author deals with 

 the resultant of two sets of stream lines and shows how 

 complicated cases can be built up out of simple cases. 

 This might have been done at first and have led up to 

 the more complicated and confusing cases which he 

 presents to the reader at the very beginning. By means 

 of a hot or cold region in the centre of a stream, he con- 

 structs stream lines which are analogous to the lines of 

 force for a dielectric cylinder in a steady uniform electric 

 field. The hot region is supposed to be produced by the 

 sun shining on this part of the stream, the rest being in 

 shadow. As a limiting case he has a region of vapour in 

 the middle of the stream, and states m a footnote : — 



"The conditions for a stationary state of flow with a 

 region of vapour in the middle of the stream through 

 which the water continuously flows are left undiscussed." 

 This part of the book is the most interesting, but it is 

 somewhat questionable whether the method would be 

 really of use to a student. 



The electrical and magnetic parts of the book are 

 clearly and, on the whole, well done, in spite of the 

 limited space at the author's disposal. The elementary 

 parts of electrostatics are very clearly and concisely 

 explained. There are some points which are somewhat 

 carelessly treated— for example, it is too much to say 

 that the process of solution of zinc in sulphuric acid is 

 completely explained by an electrolytic decomposition 

 NO. 1693, ^'OL. 65] 



due to small currents set up on the zinc producing zinc 

 oxide and setting free hydrogen, the zinc oxide then 

 dissolving in the acid. It is difficult to see how this is 

 a complete explanation. 



The last few pages are concerned with Rontgen, 

 Becquerel and kathode rays. The author here is rather 

 unfortunate, as he makes several wrong statements, e.g. 

 kathode rays charge bodies on which they fall frequently 

 with a negative, but more often with a positive charge. 

 Also Rontgen rays are stated to be probably light rays 

 of smaller wave-length than ultraviolet rays, and kathode 

 rays are stated to be probably longitudinal electric waves. 

 These statements seem extraordinary in view of the 

 Stokes-Thomson theory of Rontgen rays and the uni- 

 versally accepted emission nature of kathode rays. 



The illustrations are good, but sometimes superfluous 

 —for example, on p. 270 is an illustration of a dynamo 

 attached to a steam engine, with no description whatever 

 in the text, and the illustrations of telescope and micro- 

 scope have no accompanying description. 



On the whole, the book is very interesting and would 

 prove extremely useful to students who have already 

 been introduced to the elementary facts of the subjects. 



OUR BOOK SHELF. 

 The Principles of Mechanics. Part i. By Frederic Slate, 



Professor of Physics in the University of California. 



Pp. x-f299. (New York: The Macmillan Company ; 



London : Macmillan and Co., Ltd., 1900.) Price 



7J-. dd. net. 

 A Treatise on Elementary Dynamics. By K. A. Roberts, 



M.A. Pp. xi -f- 258. (London: Macmillan and Ca\ 



Ltd., 1900.) Price 4^. bd. 



An interesting contrast can be made between these two 

 little treatises on the methods of mathematical instruction 

 in this country and .'\merica. 



The English book is still full of elegant little calculus- 

 dodging expedients, and no difierential coefficient is 

 allowed to appear, as it is intended for candidates for 

 mathematical scholarships, who are still kept marking 

 time so long over coordinate geometry as never to arrive 

 at the calculus. 



The American professor, on the other hand, addresses 

 a class of college students of about the same age, but who 

 have brought to their task a working knowledge of the 

 calciilus, as well as a good groundwork of experimental 

 physics. This knowledge of experimental facts will en- 

 able the student to follow Prof. Slate's somewhat meta- 

 physical presentation of the subject. Mr. Roberts has 

 also incorporated the views of modern writers on the 

 laws of motion, based mainly on Mach's "Science of 

 Mechanics." This will make these treatises useful to 

 students who are to become teachers in their turn ; but 

 Prof. Perry will say that they are unsuitable for the class 

 of student he has in his mind, as the exercises and illus- 

 trations are throughout of the usual academic type, devoid 

 of reality, or else based on microphysical conceptions. 



Studien ii. d. Milchsaft v. Schleimsaft der Pflanzen. 

 Von Prof. Dr. Hans Molisch. Pp. viii-Ho9. (Jena: 

 Gustav Fischer, 1901.) 



Prof. Moi.ISCH gives a general account of the occurrence 

 of laticiferous tissue and mucilage cells in plants, treating 

 his subject specially from the point of view of the latex 

 itself He finds that the fluid is commonly acid or 

 sometimes neutral in reaction, thus differing in this 

 respect from protoplasm. He finds a curious form of 

 vesiculated nucleus to be of common occurrence, and 



