378 



NA TURE 



[August 15. 1907 



learns the dependence of this field on the nature of 

 the medium fiHinfj it, and the interdependence of 

 electrostatic and magnetic phenomena. While the 

 author in his preface directs attention to the great 

 difficulty of treating the subject in a po])ular way, 

 it is noticeable that he has been remarkably suc- 

 cessful in overcoming this difficulty, and the liberal 

 use of diagrams illustrating lines of force, as shown 

 both from plotting and by experiment, does much to 

 impart a reality to the discussions. The second 

 section is devoted to Clerk Maxwell's electromagnetic 

 theory of light, and emphasis is rightly laid on the 

 discovery of the coincidence between the " v " of the 

 physicist and the velocity of light. The third section 

 deals with Hertz's classical experiments, culminating 

 in the realisation of electric waves; while the l.-ist 

 section is devoted to those further developments which 

 have established the identity in character of electric 

 and optical phenomena and have resulted in wire- 

 less telegraphy. This section includes the study of 

 polarisation, dispersion, optical resonance, and the 

 determination of wave-lengths. The author is very 

 clever in the way he keeps mathematical formulae in 

 the background, and at the same time leaves the 

 reader perfectly conscious of their existence. 



Drs. Foppl and Abraham's two volumes deal with 

 the same phenomena treated from the point of view of 

 the advanced student of physics instead of that of the 

 popular reader. For this purpose the historic method 

 of our older books is now quite unsuitable, and we 

 are glad to see its place taken by a more powerful 

 method of treatment. The present writer has clear 

 reminiscences of the difficulties into which the text- 

 books in use in the 'eighties of last century led the 

 reader. \A'c were taught that quantities of electricitv 

 and quantities of magnetism acted on each other 

 according to the law of the inverse square, that the 

 units could be so chosen as to give unit force at 

 unit distance, and that the forces in every case were 

 derivable from a potential. We believed these state- 

 ments implicitly, and then we were introduced to 

 statements inconsistent with the first under " specific 

 inductive capacity," with the second under "electro- 

 static and electromagnetic units," and with the third 

 under "electromagnetic induction." 



Dr. Abraham's readers start by equipping them- 

 selves much better with the analytical methods re- 

 quired for arriving at a clear conception of electro- 

 magnetic phenomena. The first section, extending 

 to 122 pages, written by Dr. A. Foppl, deals exclu- 

 sively with vector analysis and vector fields, and the 

 latter subject is introduced by means of hydro- 

 dynamical analogies. It may truly be said that if 

 the study of the dynamics of perfect fluids has not 

 led to many practical applications, it possesses great 

 educational value in affording a simple and intelligible 

 representation of the quantities and equations occur- 

 ring in electromagnetism. By the time the student 

 has grasped these introductory notions he is able 

 easily to follow the sections on electrostatics and 

 magnetostatics without falling into any misconcep- 

 tions regarding their limitations, and he can then 

 pass on directly to the study of electromagnetic induc- 

 NO. T972, VOL. 76] 



tion and electric waves. Especially careful is the 

 author in avoiding the old pitfalls in connection with 

 the " Dimensions of Units." His equations are all 

 written with the three constants, and he then shows 

 how the electrostatic, electromagnetic, and Gauss's 

 systems of units may be obtained by putting two but 

 of these three constants equal to unity. U'e should, 

 however, have liked to see the dimensions of the units 

 tabulated in terms of force (or perhaps energy), 

 length and time instead of mass, length and time. 

 The formuhe are then much simpler, and better 

 express the fact that electrical phenomena in general 

 manifest themselves to our senses by the forces they 

 produce on material bodies, though the existence of 

 these phenomena is independent cif the concept of 

 mass as commonly understood. 



So much for the first volume, which, as the author 

 points out in his preface to the second volume, covers 

 what may be described as the " first floor " of modern 

 electrical theory. The second floor, which has now 

 been built up as a superstructure on the first, con- 

 sists of the modern electron theory discussed in this 

 second volume. 



The first chapter deals with the definition of an 

 electron and its elementary properties, while the 

 second deals with radiation from electrons. In 

 chapter iii., which deals with the dynamics of a 

 system of electrons, certain assumptions have to be 

 made. In most cases Dr. .\braham follows LxDrentz's 

 theorv. In Ihe second section, in dealing with pon- 

 derable bodies in motion, comparisons are made 

 showing the differences in the results obtained by 

 Lorentz's and Hertz's assumptions. 



In a book like the present, dealing with theories 

 about which differences of opinion exist, the list of 

 formulaj collected for reference at the end is particu- 

 larly useful, as it enables the reader to see at a glance 

 wliat hypotheses are made. While the book contains 

 an excellent exposition of the electron theory, based 

 on these hypotheses, we only have to turn to the table 

 at the end of Dr. Witte's book to find th it there are 

 twelve different concei.vable mechanical theories of 

 electric phenomena of which the great majority are 

 classed as recognised theories. 



Of these theories, two are theories of action at a 

 distance, and the third is Newton's emission theory, 

 all of which are easily dismissed as insufficient to 

 account for known phenomena. The remaining 

 theories, which Dr. Witte describes as undulation 

 theories, and their classification is a simple matter 

 of permutations and combinations. W"e have two kinds 

 of energy of the ether, electrostatic and electro- 

 magnetic, which have to be potential or kinetic, or 

 partially potential and partially kinetic. These possi- 

 bilities lead to six different combinations or nine 

 different permutations. The cases where each part of 

 the energies is wholly kinetic or wholly potential lead 

 to three combinations, namely, Mie's group (both 

 energies potential). Lord Kelvin's (one potential and 

 one kinetic), and Hertz's (both energies kinetic). But 

 another totally independent line of classification is 

 also pointed out in the separation of theories of the 

 ether into atomic hypotheses and hypotheses of a con- 



