142 



NATURE 



[March 30, 191 1 



Ihc Glacial period, especially as compared with the 

 fauna of tlu- Alps. Another chapter deals with the dis- 

 tribution of niaminals, rii)tiles, amphibia, moUusca, 

 &c., in the Carpathians, followed by lists of Coleoptt-ra 

 (by Holdhaus and Deubel) found in different districts 

 in the Carpathians, with notices of the surroundings. 

 Ihe niajj illustrates the glaciation of the eastern Alps 

 and Carpathians during the Ice period, and the range 

 of the blind mountain beetles. 'Ihese beetles are a 

 specially interesting group, of limited range, in the 

 Austrian Alps. Some of them are cave species, while 

 others inhabit the open. 



Many interesting subjects are discussed by Prof. 

 Holdhaus, which we have no space to allude to, but he 

 has iiot forgotten to take account of fossil and amber 

 Coleoptera, and his remarks on what he calls " Massifs 

 de refuge " (districts south of the Alps to which he 

 believes the mountain species retreated during the 

 Glacial period) also seem to deserve special attention. 

 Mosses and Liverworts. An Introduction to their 

 Study, with Hints as to their Collection and Pre- 

 servation. By T. H. Russell. New and revised 

 edition. Pp. xvi + 211 + xiii plates. (London: 

 Sampson, Low, Marston, and Co., Ltd., 1910.) 

 Price 4.V. 6d. net. 

 The speedy demand for a second issue of Mr. Russell's 

 book testifies to its value and usefulnes as a guide 

 to the study of mosses and hepatics. The introduc- 

 tory portion is all that can be desired, and the student 

 who conscientiously masters this portion will be in a 

 position to pass on to more advanced books dealing 

 with the same subject. The author lays much stress 

 on the point that he is specially anxious to use simple 

 language, fearing that scientific words might act as a 

 deterrent to the study. With this frequently expressed 

 idea we do not quite agree ; the true value of a 

 scientific term consists in the fact that, when once 

 grasped, it stereotypes the particular structure in a 

 single word, whereas a sentence in English may convey 

 but a very vague idea of the structure in question. 

 As an example, the term archegoniiim defines a 

 definite structure, which is said to be " the fruit-bearing 

 organ," which it certainly is not. The species given 

 as e.xamples are well chosen, and cover all the struc- 

 tures peculiar to mosses and hepatics. 



The detailed account of habitats, and the stress laid 

 on their inipoitamc, are features to be commended, as 

 too frequent 1\ ihc student is encouraged to snatch a 

 fragment from anywhere, put it into a tube containing 

 methylated spirit, and only commence serious study 

 when viewing it under a compound microscope. The 

 chapters on collecting, storing, and the preparation of 

 mounted specimens for microscopic work are very full, 

 and are obviously the outcome of much practical 

 experience. 



Thirteen whole plates of excellent figures add much 

 to the value of the book, which can be confidently re- 

 commended as a stepping-stone to the study of mosses 

 and hepatics. 



The Social Guide, iqii. Edited by Mrs. Hugh Adams 

 and Edith A. Browne. Pp. xxxviii + 252. (Lon- 

 don: A. and C. Black, 1911.) Price 2s. 6d. net. 

 Information is given in this work of reference not 

 only about occupations for leisure days .ind hours, 

 but also concerning more serious pursuits. The 

 diary with which the volume begins includes the meet- 

 ings of the Royal Geographical Society and of the 

 British Association among scientific societies, and 

 the anniversary dinner of the Royal Society. Though 

 it is sometimes a little difficult to understand the 

 principle of selection for the contents of the volume, 

 we notice the activities of the Royal Institution, the 

 Roval Society of Arts, and the Zoological Society are 

 described. 



NO. 



2 161, VOL. 86] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opim 

 expressed by bis correspondents. Neither can he underlaiiH 

 to return, or to correspond with the writers of, rcjectiA 

 manuscripts intended for this or any other ^arto/ Nat 1 

 No notice is taken of anonymous communications.] 



A Kinetic Theory of Gravitation. 



The subject of Mr. Brush's article in Nature ol 

 March 23 (p. 129) is certainly of profound interest, and 

 will continue to be so until the problem as to the nature 

 of gravitation is solved. Meanwhile, a few questions 

 raised are comparatively simple. Anyone asked, Where 

 lies the energy of a raised weight? must surely reply, '* In 

 the a:ther," i.e. in the medium, whatever it is, that i» 

 driving the weight down towards the earth. A critic who 

 either doubts or asserts this will not be confused — as 

 Mr. Brush suggests he will be — by the suggestion that 

 the weight might be raised so high as to reach the 

 neutral point between earth and moon — a suggestion 

 which carries with it the tacit questions, " Where i» 

 the energy now?" and " What has become of the work 

 done?" — for this case is no more troublesome than the 

 case of a weight raised and hung on a hook. Something 

 — some opposition force — sustains the weight, i.e. opposes 

 the pull of the earth, and it matters little whether the 

 opponent be a shelf beneath it or the moon above it. The 

 important thing to understand is the nature of the down- 

 ward propelling force — indeed, of both the upward and the 

 downward force — in either case. 



The question whether the energy of a raised weight is- 

 potential or kinetic is of little or no importance. The 

 energy is certainly potential, according to our definition 

 of potential. So is the energy of a strained spring : for 

 there also the atoms are separated against their mutual 

 (cohesive) attraction, and there again the energ>- really 

 resides in the aether. But that all energy may turn out 

 to be ultimately kinetic — when we come to understand 

 what elastic stress fundamentally is — that proposition is 

 not negatived in the least. 



Mr. Brush proposes a shadow theory of gravitation, a 

 modification of Le Sage's theory except that the pressure 

 is supposed due to the non-syntonic impact of waves 

 travelling in all directions, instead of to a bombardment 

 of utterly minute particles flying at random. There is 

 nothing new in a shadow theory, and all such theories are 

 faced with the difficulty of plausibly explaining the absence 

 of noticeable screening — a difiicuity which is bound to 

 reduce them to acquiescence in an approximation. 



The contribution which Mr. Brush makes to the dis- 

 cussion is the suggestion that the supposed gravitational 

 rether-waves are the result of accumulated thermal radia- 

 tion from all past and present suns, the wave-lengths- 

 having automatically increased during their long storage. 



To this view several objections might be urged — one of | 

 them being that in that case the constant of gravitation 

 would be secularly increasing ; another, that it should be 

 greater in a hot enclosure, say the interior of a sun. than 

 elsewhere ; but a more salient obstacle is raised by the 

 inquiry as to which is cause and which is effect. How 

 did the bodies get hot and so radiate? Was not their 

 heat perhaps due to their having clashed together with 

 gravitational energj- itself derived from the aether? 



The fact is that every question concerning origin involves 

 us always in insuperable puzzles, and that is just the main 

 difficulty about gravitation. An atom of matter, by its 

 very existence, sets up a fixed stress in the aether, vary- 

 ing directly as the mass and inversely as the distance — 

 that is only another way of stating the law of gravitation r 

 we are trying to understand the nature and cause of that 

 stress. It appears to be one of the fundamental properties 

 of matter, and until we can understand what is meant by 

 the generation or destruction of an .itom — i.e. of an 

 electron if that is the fundamental unit — we are hardly 

 likely to understand its gravitational influence more than 

 any other of its fundamental properties — including, per- 

 haps, existence itself. 



Let this not be understood as a negative prediction or 

 estimate of impossibility — such predictions are always 

 absurd ; it may be that when the structure of an electron 



