558 



NA TURE 



[April 12, 1900 



intended to be sufficient to enable them to secure accurate 

 results. It is remarkable that this combination of the 

 theoretical and the practical is not more often attempted. 

 A book on any branch of experimental physics necessarily 

 contains so much description of experimental work, that 

 the additional instructions necessary to enable the 

 student to carry out the experiments for himself would 

 not add greatly to the length of the book. In a similar 

 manner, a book intended simply for practical work in the 

 laboratory generally and necessarily contains so much 

 theory that it would not be a difficult matter to include all 

 that the student would be likely to require in this respect. 



The limitations which the author has imposed upon 

 himself with regard to the use of the calculus apply chiefly 

 to the section on thermodynamics. The method of ex- 

 pansion by the binomial theorem is used instead. The 

 proofs are worked out from first principles, and are there- 

 fore generally longer than they would have been if the 

 methods of the calculus had been assumed. But as a 

 compensation they are much more instructive. The 

 method of proof compels a close attention to each detail 

 of the work, which is likely to result in a much clearer grasp 

 of the physical meaning of the equations than the 

 mechanical performance of mathematical rules for differ- 

 entiation or integration. The student who has failed to 

 follow the purely geometrical treatment of the same sub- 

 jects in Maxwell's " Theory of Heat," will probably find 

 these sections extremely helpful. They may be also 

 strongly recommended to the mathematical student who 

 does not desire to regard physics merely as a mathematical 

 exercise. 



In the selection of the experiments to be performed by 

 the student, the author appears to have erred on the side 

 of making them too simple, and of not exacting a suffi- 

 ciently high standard of accuracy of the advanced student. 

 He very rightly lays great stress on the importance of 

 accurate thermometry, on which nearly all experiments 

 depend. He might with advantage have given some 

 details of the " variable zero method " of employing 

 mercury thermometers, so ably expounded by Guillaume, 

 which is now so generally used for accurate work. The 

 method is laborious, but possesses undoubted advantages, 

 and ought to be described in an advanced text-book, espe- 

 cially as it does not present any great theoretical difficul- 

 ties. In determining the expansion of glass by means of 

 the mercury weight thermometer (p. 68), the advanced 

 student should be instructed to use the accurate equation 

 g={tn - ;«^)( W —w)lW, instead of the approximate equa. 

 tion^=w -;«;,, which is given in nearly all the text-books. 

 He should not be permitted to make an error of i6 per 

 cent, in his calculation, when he may easily obtain ob- 

 servations correct to a tenth of i per cent. Similarly, in 

 a text-book for advanced students, it would be more 

 instructive if the author, in introducing a description of 

 some of the old time-honoured experiments, had ventured 

 to be a little more critical of their weak points, and to 

 explain why they failed, or in what respect the deductions 

 made from them were uncertain, or how they could be 

 improved. It seems a pity at the present day, for instance, 

 to repeat Tyndall's fairy tales about the absorption of 

 heat by vapours without adding a large proportion of 

 salt. 



NO. 1589, VOL. 61] 



A special section is devoted to electrical thermometry, 

 including an explanation of the principles of the methods 

 employed, which is simple and at the same time fairly com- 

 plete so far as it goes. We may note, however, in passing, 

 that a platinum thermometer cannot in general be calibrated 

 by reference to the absolute zero, as the resistance of the 

 pure rrietal " tends to vanish " at a much higher temper- 

 ature [Phil. Mag. Feb. 1899). Also that if a reasonably 

 sensitive galvanometer is used, the heating effect of the 

 current ought not to exceed a hundredth of a degree. 

 The section contains an account of the thermocouple, 

 thermopile, radio-micrometer and bolometer, which 

 should be useful as well as interesting. In other subjects 

 also, such as the liquefaction of gases, the book appears 

 to be well up to date within the limits which the author 

 has set himself. The whole arrangement is extremely 

 clear and practical, and well adapted to meet the needs of 

 students, who will find the most important points distinctly 

 emphasised. There is a useful summary at the end of 

 each chapter, and an excellent collection of examination 

 questions. Considering its small size, the book contains 

 a remarkable amount of information. 



In the preface of Prof Lommel's " Text-Book " the 

 following explanations occur : — " The present text-book 

 has grown out of the author's lectures, and is intended 

 to develop the subject on an experimental basis in such 

 a manner as to make the book easily accessible to be- 

 ginners. But in order to meet the needs of higher schools 

 and colleges, paragraphs in ' fine print ' are interspersed, 

 which contain the most important mathematical develop- 

 ments in terse and simple form. The author, as a 

 general rule, has employed pure German words rather' 

 than technical expressions from foreign tongues, e.g. 

 wucht instead of energy of motion or kinetic energy, 

 and spannung instead of potential and difference of 

 potential. The translator has not preserved the author's 

 distinction between ' potential ' and ' tension ' {spannung), 

 but has otherwise attempted only a faithful and worthy 

 reproduction of the original." 



These aims and endeavours on the part of the author 

 and translator appear as a possible explanation of the 

 introduction of several rather unfamiliar terms to English 

 readers, such as " living force " for kinetic energy, " laws 

 of shock" for impact, "stretch" for extension, "melting 

 heat " for heat of fusion, " overmelted " for superfused, 

 and similar phrases. The word " tension " certainly seems 

 to be rather overworked, as it is used for the pressure 

 of gases and vapours, as well as for electric potential, and 

 even in one place for energy of position. On the other 

 hand, we observe the apparently needless introduction of 

 such words as " gyrotrope," " pachytrope," " rheotome," 

 " rheometer " for the more familiar commutators, switches 

 and galvanometers. The frequent use of oxygen for 

 hydrogen, calcium for potassium, and coal for carbon are 

 possibly simple errata. 



The author has endeavoured to follow the historical 

 order as being the most natural and interesting in the 

 development of each part of the subject. The names and 

 dates introduced in following this plan are often instructive 

 and show a greater familiarity with English work than is 

 common in Continental text-books. We may instance the 

 dates, Boyle 1662, Mariotte 1679. The author neverthe- 



