404 



NATURE 



[November 25, 1920 



schools shows that there is a demand for this 

 knowledge. We think, however, the author has 

 included too much in the scope of the work. We 

 read about mag-netism, primary batteries, electric 

 bells, and kinema sets. We also read about 

 three-core cables, rotary converters, boosters, 

 interpoles, etc. The book would have been more 

 useful if the description and elementary theory of 

 the more intricate apparatus had been excluded. 

 It does not advance our technical knowledge of 

 what is meant by "candle-power" to be told that 

 "a source of light is said to possess candle- 

 power." It is also not very instructive to be told 

 that the back E.M.F. of a motor can be obtained 

 by Fleming's right-hand rule. We failed to follow 

 the theory given for the action of the balancers in 

 a three-wire system of distribution (p. 246). The 

 reader ought to be told why the difference of pres- 

 sure between the two ends of a circuit is called 

 the "potential difference." The introduction of 

 the word "potential" must strike him as mysteri- 

 ous. In electrical science, more almost than in 

 any other, it is impossible to be perfectly exact 

 "at once," but a beginning at precision should 

 be made early, even although the author should 

 run the risk of being called "academic." 



The Nomenclature of Petrology: With References 



to Selected Literature. By Dr. Arthur Holmes. 



Pp. V -1-284. (London : Thomas Murby and Co., 



1920.) Price 12S. 6d. net. 

 There is probably no department of science in 

 which the nomenclature is in such a state of con- 

 fusion as in petrology. The rocks that form the 

 earth's crust include an innumerable variety of 

 types which pass by imperceptible degrees into 

 one another, so it is not surprising that rock 

 names have been multiplied to an extraordinary 

 degree, and that there has been considerable varia- 

 tion in their application. The student of petro- 

 logical literature, therefore, frequently finds him- 

 self faced with unfamiliar terms or those which 

 are used in a sense different from that with which 

 he is acquainted, and he will owe a debt of grati- 

 tude to Dr. Holmes for the labour expended in 

 compiling this invaluable work of reference. He 

 will no longer be dismayed when he meets in the 

 pages of geological publications with "lavialites," 

 "ledmorites," "leeuwfonteinites," and "leid- 

 leites," all of which are explained in a single page. 

 The book deals not only with the names of rock 

 types, but also with those indicating their struc- 

 tures and other characters, and we shall know now 

 what is meant when a rock is referred to as being 

 " lepidoblastic " or "glomeroplasmatic." 



A useful feature is a list of the commoner pre- 

 fixes and suffixes and the meanings usually at- 

 tached to them by petrologists, and there is a 

 glossary of French and German terms. A tabular 

 classification of rocks, including ore deposits, 

 follows, and presents many novel and interesting 

 features. It is based partly on fundamental prin- 

 ciples of rock genesis, and so far it is likely to 

 hold its own in the future ; and partly on arbitrary 

 numerical criteria, and must to that extent be con- 

 NO. 2665, VOL. 106] 



sidered only a provisional stage in the evolution 

 of a scientific classification. There is an instruc- 

 tive synopsis of processes of alteration due to 

 igneous exudations, and of their products ; and the 

 classification of metamorphic rocks according to 

 their structure into maculose, schistose, gneissose, 

 and granulose appears to be distinctly useful. 

 . ^ J. W. E. 



Liquid Air and the Liquefaction of Gases. By 

 Dr. T. O 'Conor Sloane. Third edition, revised 

 and much enlarged. Pp. 394. (London : Con- 

 stable and Co., Ltd., 1920.) Price 21s. 



It must be confessed that the raison d'etre of this 

 book is not easy to discover. From its sub-title 

 ("A Practical Work," etc.) — and, we may re- 

 mark, from its price — one might expect an 

 authoritative book of reference for the engineer. 

 It is, however, intended as a popular exposition 

 of the history of the liquefaction of gases. 



An introduction to the elementary facts of 

 physics is followed by a series of chapters on the 

 personalities, methods, and apparatus of some 

 leading experimenters, beginning with the work of 

 Faraday, and culminating in the achievements of, 

 Mr. Charles E. Tripler. An outline is given of 

 the Linde, Hampson, and Claude processes. 

 Numerous experiments, such as that of pouring 

 liquid air oi^ the floor, or boiling it on a block 

 of ice, are described and illustrated. Indications 

 are given of some applications of liquid air. 



The author has evidently read with care much 

 of the historical literature, and his digests of 

 some of the early work are well told. He would 

 probably not lay claim to a first-hand acquaint- 

 ance with the scientific and industrial cryogenic 

 developments of the twenty years which have 

 elapsed since the book was first written ; and it 

 may be doubted whether the information imparted 

 is of a kind to satisfy any but the most super- 

 ficially minded of readers. 



George Stephenson. By Ruth Maxwell. (Heroes 

 of All Time.) Pp. 192. (London : George G. 

 Harrap and Co., Ltd., 1920.) Price 3s. 6d. net. 



An account is given in this book of the more 

 important events which marked the career of 

 George Stephenson. A few pages are devoted 

 to a brief account of his childhood and early 

 struggles. Then comes an account of the in- 

 vention of the "Geordie" safety lamp for use in 

 mines, which was brought out simultaneously 

 with Sir Humphry Davy's famous lamp. The 

 remainder of the book records in detail the more 

 interesting points in the history of the great 

 engineer from the time when he built the Stockton 

 and Darlington Railway onward, and it is amus- 

 ing to read of the struggles he had from time to 

 time to secure the requisite Parliamentary 

 authority for building railways on which traffic 

 would travel at ten miles an hour ! Nine excel- 

 lent full-page illustrations showing some features 

 of British railways in Stephenson's time make 

 an agreeable addition to an interesting book. 



