NA TURE 



[November i8, 1909 



scription of it is not given until p. 276, where no 

 reference to the foregoing figure occurs. The same 

 dislocation of figures occurs in many groups. 



(2) Mr. Crawford is an impressionist, and one who 

 shares the Wordsworthian spirit. In these slight 

 word-pictures of east-coast scenery he sketches, in a 

 very dainty and observant fashion, the picturesque 

 features of animal life as seen against the varying 

 background of nature's moods. He believes, and 

 reiterates his belief in, the value of our response to 

 such beauty. He is an artist. 



"We think in pictures. We recall in pictures. We 

 remember more than we see. With this aid, to evolve 

 our artistic sense were easy, to add a touch of 

 imagination were not hard. Thus, simply, might the 

 world be made to seem beautiful and life be filled 

 with gladness." 



These essays will perhaps not add to Mr. Craw- 

 ford's reputation, but they will certainly not detract 

 from it. 



(3) This little book is mainly a reprint of articles 

 on the geology of Melbourne and neighbourhood con- 

 tributed to the Melbourne Argus, but also includes a 

 survey of the strata round Port Phillip Bay and up 

 country to Ballarat and Bendigo. The bedrock, of 

 Silurian age, is characterised by rough, dark-barked 

 gum-trees, and is economically of great importance 

 for the making of pottery and bricks. The wearing 

 down of this rock has produced the gravel beds in 

 which the gold of the bedrock is found. These gravel 

 beds have also their characteristic flora, the grass- 

 trees. Intersecting these are granitic rocks which 

 have w-eathered out, and support a flora of native 

 cherries, white-barked gums, and other trees. The 

 vast blue-stone plains have been formed by eruptions 

 of lava emerging through the bedrock, and are dis- 

 tinguished by their grassy, treeless appearance. In 

 addition there are glacial beds in many districts, and 

 cinder beds in a few others, supporting dense forest 

 and forming rich farm land. The descriptions are 

 very clear, and are illustrated by good photographs. 



ELECTROMAGNETIC THEORY. 

 (i) Anfangsgriiiide der Maxwelhcheii Theoric, ver- 



kniipft mil dcr Elektronentheorie. By Franz 



Richards. Pp. i.\ + 245. (Leipzig: B. G. Teubner, 



1909.) Price 7 marks. 

 (2) The Theory of Electrons, and its Apjilications to the 



Phenomena of Light and Radiant Heat. By H. A. 



Lorentz. Pp. iv + 332. (Leipzig: B. G. Teubner; 



London : David Nutt and Williams and Norgate, 



1909.) Price 8 marks. 

 (O'T'HE first of these volumes deals with the 

 ■*■ foundations of Maxwell's electromagnetic 

 theory. The author is careful to point out that it is 

 not a text-book, but a sketch based on lectures de- 

 livered to teachers. For this reason the treatment 

 differs somewhat from what may be regarded as the 

 normal method of presenting the subject to students; 

 although in a branch of physics which appeals to a 

 comparatively limited class, it may be doubted whether 

 there are satisfactory grounds for introducing pre- 

 ferential treatment. 



NO. 2090, \'OL. 82] 



At an early stage the author shows how the two 

 fundamental "circuital relations" are connected by 

 means of the principle of least-action. This is an ex- 

 cellent feature, and the proof would probably have 

 produced greater impression had some definite physical 

 picture, such as that adopted by Larmor, been intro- 

 duced. 



In succession, the author treats of static effects in 

 non-conductors, of conduction, and of magnetic effects 

 of currents. The introduction of Stokes' theorem, 

 which is delayed until this stage, might, with advan- 

 tage, have been introduced much earlier. 



Induction is next discussed, and finally there is a 

 chapter on electromagnetic waves, which closes with 

 a brief and rather imperfect treatment of metallic 

 reflexion. 



As a whole the book is somewhat disconnected, but 

 it ought to be judged as a collection of monographs, 

 and from this point of view the treatment is clear 

 and good. 



(2) Lorentz' book deals with the latest development 

 of the electromagnetic theory. It contains a series 

 of lectures delivered in Columbia L'niversity, New 

 York, in 1906, and will be welcomed as his latest 

 views on a subject which owes its origin and much 

 of its development to Lorentz himself. 



There are five cha|)ters and a section of notes, which 

 give calculations too elaborate to be included in the 

 text. Chapter i. treats of general principles and the 

 doctrine of free electrons. It is to Lorentz that we 

 owe the view that the free eether is to be regarded as 

 at rest, and that hence phenomena in it are governed 

 by the two "circuital" and the two " solenoidal " 

 equations of Maxwell. 



At discrete points we may have electrical singular- 

 ities characterised by a certain density. The effect of 

 this at those points is simply to make the electric 

 divergence equal to the density, instead of nil, and 

 to add to the displacement current, a portion due to 

 convection of amount equal to the product of density 

 and velocity. -Vt other points in the aether the effects 

 are sufficiently included in the four fundamental equa- 

 tions. It is of importance to notice that this specifi- 

 cation imposes a limitation on the mtcrnal character 

 of an electron. Thus if an electron is defined as a 

 shell with a surface charge of electricity, its interior 

 must, on the basis of Lorentz' equations, consist o{ 

 nothing but Kther. The limitation carries important 

 consequences, such as prescribing surface conditions. 



In this chapter the author discusses the question of 

 electric inertia on the basis of Kaufmann's experi- 

 ments ; but as he again takes up the question more 

 fully in a later chapter, it will be convenient to reserve 

 our observations. 



Chapter ii. is devoted to the question of radiation 

 and absorption of heat. Those who have followed the 

 interesting discussion on this matter in the 

 Physikalische Zeitschrift recently, will not be sur- 

 prised to find that Lorentz devotes some space to the 

 question whether the tether and the radiating body, 

 supposed to consist of electronic radiators, can be re- 

 garded as a system to which the law of equipartition 

 of energy can be applied. The result of such an as- 

 sumption is in flat contradiction to observation. As 



