August 26, 1909] 



NA TURE 



245 



Sbook should give incorrect or misleading informa- 

 tion ; the authors' aim " at spanning the gulf which 

 too often divides pure theory and practical engineer- 

 ing " will not be realised if the student is obliged 

 to unlearn much that they teach him when he 

 becomes a practical engineer. We do not profess to 

 ibe experts in the whole subject of electrical engin- 

 eering, and cannot criticise the whole book, therefore, 

 'On the same lines as we have criticised the section 

 on lamps ; but the authors, by writing such a book, 

 lay claim — at least so far as fundamentals are con- 

 cerned — to be such experts, and if we find them at 

 fault at one part we are led to suspect the whole. 



The book covers the whole electrical field; the 

 arrangement is that usually adopted, opening witli 

 electrostatics and magnetism, and passing on to 

 .electric currents. The diagrams and illustrations 

 are for the most part good, but the process blocks 

 i(fortunately few) come out badly on the class of 

 paper used. There are numerous exercises for the 

 student t^ work out at the end of each chapter. 



M. S. 



OVR BOOK SHELF. 



■Gas-engine Theory and Design. By A. C. Mehrtens. 

 Pp. v + 256. (New York: John Wiley and Sons; 

 London : Chapman and Hall, Ltd., 11)09.) Price 

 los. 6d. net. 



The writer of this book is an instructor in 

 mechanical engineering in the Michigan Agricul- 

 ttural College. His aim, he tells us, has been to 

 prepare a book for all who are interested in gas 

 engines, whether students, draughtsmen, engineers, or 

 engine operators. 



This is an ambitious aim, and we may well doubt 

 the possibility of its being carried out in such a small 

 •compass ; but there can be no doubt that the cardinal 

 •virtues of simplicity and conciseness of language which 

 any such intention must require are here presented in 

 ■no usual degree. The reviewer does not remember 

 any book hitherto written on the gas engine which 

 presents its subject with such lucidity. 



The chief entry to be made on the debit side of the 

 account is that the extent of the field covered is far 

 too great. It will be found, on perusing the volume, 

 that it not only deals with the history and present 

 position of gas-engine invention, and with the pro- 

 perties of the gases and fuels used, but also with 

 such a big subject as the design of engine details and 

 the dimensions of parts. Students usually learn their 

 physics and machine design independently of the 

 steam or gas engine, and a book on the gas engine 

 which includes a great deal of what has already been 

 •studied separately is wasting space. The result in 

 so small a book as this is that the truth and ap- 

 plicability of a great number of formulce are taken 

 for granted, which may account for the poor 

 compliment paid to them by the author on p. 123, 

 where he remarks : — 



" A number of formulas will be given in the follow- 

 ing paragraphs, but machinery cannot be designed by 

 formulas alone. The author has frequently found 

 that empirical, and other, formulas would sometimes 

 come within 500 per cent, of the correct result." 



There are also the inevitable slips of a " first 

 edition," but they are not numerous. The author 

 should, however, make a point in the next edition of 

 correcting his description (on p. 33) of carbon mon- 

 oxide as an unstable compound; his omission on p. 39, 

 •NO. 2078, VOL. 81] 



in the discussion of the apparent suppression of heat 

 on explosion, of any reference to the increase of specific 

 heats admitted on p. 25 ; the error in saying (on p. 44) 

 that it is usual to increase the compression pressure 

 in an engine which is to run on kerosene, and he 

 should also correct the general confusion of the table 

 on p. 167. It is difficult to understand what the author 

 means in his description (on p. 52) of the •working 

 of the gas producer by the remark : — " The limit of 

 the ratio of steam to coal by weight is about i to 40." 



Although, as has been stated, the author has at- 

 tempted to get too much into so small a volume, it 

 must be acknowledged that he has produced a book 

 at once interesting in treatment and clear in language. 



La materia radiante e i raggi magnetici. By Prof. 



.\. Righi. Pp. vii + 308. (Bologna : N. Zanichelli, 



1909.) Price 8 lire. 

 In a recent number of Nature a brief account •was 

 given of Righi's " magnetic rays," this being the 

 name applied to a peculiar luminosity near the 

 kathode of a vacuum tube, when the latter is placed 

 in a longitudinal magnetic field. Righi supposes 

 that this luminous column is due to electrically 

 neutral doublets, which are not in sufficiently stable 

 equilibrium to be looked upon as atoms or mole- 

 cules, which owe, in fact, such stability as they 

 possess to the action of the magnetic field. Several 

 papers on this subject have been published by the 

 author, and the main object of the present small 

 volume is to give a connected account of the whole 

 research. About one-third of the book is devoted to 

 an extremely lucid and interesting summary of our 

 present knowledge concerning the corpuscular theory 

 of matter, written in a style which, as far as_ pos- 

 sible, is free from technical terms. The remainder, 

 except for three short mathematical appendices, deals 

 with the evidence for and against the existence of 

 neutral doublets or magnetic rays. Here, while very 

 suggestive, the experiments are not altogether con- 

 vincing — this is evidently the opinion of Prof. Righi 

 himself — but this is due in great measure to the 

 difficult experimental conditions, ^^'hile no one ex- 

 periment can be said to have demonstrated the 

 existence of magnetic rays, the results as a whole 

 certainly tend to support the author's view. One 

 point might have been treated more fully, viz., the 

 conditions under which a magnetic field lowers the 

 potential difference at the terminals of the discharge 

 tube. Experiments are described, in some of which 

 an increase, in others a decrease, of potential is 

 brought about by the magnetic field, but it is not 

 clear to what difference in the conditions this is due. 



R. S. W. 



Brassolidae. Bv Dr. H. Stichel. (Das Tierreich, 25 

 Lieferung.) Pp. xiv-i-244. (Berlin : R. Friedlander 

 und Sohn, 1909.) Price 15 marks. 

 This is a very elaborate monograph of a compara- 

 tively small group of butterflies found only in 

 Tropical America. They form a subfamily of the 

 great family Nymphalida, and are most nearly 

 allied to the great blue iMorphidae, but differ from them 

 by their stouter bodies, darker colours, and the 

 closed cell of the hind wings, which are generally 

 ornamented with three large eye-spots on the under- 

 surface. Their flight is crepuscular, while that of 

 the Morphida; (which are represented in the East 

 Indies as well as in Tropical .America), is diurnal. 



In 1823, Latreille and Godart, in the second part 

 of " Papillons " in the " Encyclop(5die methodique," 

 were acquainted with only twenty-three species now 

 referred to the Brassolidae. Of these, twenty-one 

 formed the bulk of the second section of the genus 

 Morpho, while the remaining two species were 



