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of quantity of electrification and potential, apart from any particular law of force 

 or any mathematical formulae. The gain to the student in comprehension is 

 enormous. When he is made familiar with an experimental arrangement by 

 which one can, as it were, " ladle " multiples and submultiples of a definite charge 

 into a body, he begins to feel almost as intimate with a unit of electricity as with a 

 pint of water. If we had any criticism at all to urge on this part of the work, it is 

 that, considering the analogy drawn between potential and level, the definition of 

 potential given on page 74 of Chap. VI., a definition notorious as a stumbling 

 block to the student, is not consonant with the spirit of this chapter, and might 

 well be replaced by some such phrase as: "the potential is a scalar quantity 

 whose gradient in any direction is equal and opposite to the field intensity in that 

 direction." 



One of the noteworthy features of this volume is the full treatment accorded 

 to certain parts of the subject which hitherto have been ignored or but briefly 

 mentioned in the majority of English text-books. We have here very adequate 

 accounts of experimental methods which have been employed to measure the 

 specific inductive capacities of different types of material, of the electrification 

 produced by heating or straining certain crystals, of the measurements carried out 

 in recent years by Curie, Wells, Townsend, and Pascal on the susceptibility of 

 paramagnetic and diamagnetic bodies, and of Langevin's and Weiss's developments 

 of Ampere's molecular hypothesis. A point of departure from usual methods of 

 exposition consists in the early introduction of the student to the close connection 

 between the phenomena of electricity and magnetism and those of light. There 

 is also as complete a development of Maxwell's expressions for the stress in an 

 electric field as can be obtained without the use of advanced analysis ; and as 

 an addition to it a chapter dealing with Quincke's and Kerr's work on the elastic 

 strains in a dielectric accompanying an electrostatic field in it. Thus is the 

 danger of the student confusing Maxwell's hypothetical " displacement " or 

 "electric strain" with an elastic strain of the usual type considerably obviated. 

 The volume is to be heartily recommended, not only for the mass of information 

 contained in it, but also for the attitude which will be induced in the mind of the 

 thoughtful student by a study of its pages. 



The Spectroscopy of the Extreme Ultra-Violet. Monographs on Physics. 

 By Dr. T. Lyman, Ph.D. [Pp. v + 135.] (London : Longmans, Green & Co., 

 1914. Price $s. net.) 



This book deals with the investigations which have been carried out in the region 

 of the spectrum extending beyond the wave-length 2,000 Angstrom units, and 

 called the " Schumann region," on account of the extensive contributions to our 

 knowledge of it made by Victor Schumann. Observations of the first " octave " 

 of the ultra-violet, 4,000 to 2,000 A.U., are possible with apparatus in which 

 lenses, prisms, etc., are made of quartz or uviol glass, and from which it is not 

 imperative that air should be excluded. Above this " octave," however, it becomes 

 necessary to replace quartz by fiuorite, especially the clear, colourless variety, and 

 to mount the apparatus in a vacuum chamber. Recognising this fact, and being 

 possessed of unrivalled powers of manipulation, Schumann constructed a vacuum 

 spectograph with which he was able to reach a limit estimated to be about 

 1,000 A.U. Owing, however, to insufficient data concerning the dispersive 

 power of fiuorite, accurate measurements of wave-lengths could not be made. At 

 Harvard University, Dr. Lyman has carried out a series of researches with a 



