198 



NATURE 



[May 4, 1916 



have been taken up. Yet it does lead to conclu- 

 sions which impose serious limitations on the con- 

 ditions under which statistical energy equilibrium 

 is possible. It shows that a given distribution of 

 density of the co-ordinates of a system in statis- 

 tical equilibrium can only possess a definite 

 amount of kinetic energy; that such a state of 

 equilibrium may be stable or unstable; that cer- 

 tain distributions are incompatible with statistical 

 energy equilibrium because they would give a 

 negative value for the squares of the velocity 

 compounds, and in particular that statistical 

 energy equilibrium, such as occurs in the mole- 

 cules of a gas, is impossible in a system of bodies 

 attracting each other according to the Newtonian 

 law of gravitation. It is quite likely that such an 

 investigation if continued would lead to the deduc- 

 tion of a system the energy of which might have 

 one or more of a series of discrete values, and 

 might not be capable of continuous variation, or 

 again of a system possessing a large number of 

 discontinuities in the amount of energy which it 

 could contain. It is scarcely probable that the 

 amounts of energy would be proportional to the 

 numbers i, 2, 3, . . ., but we imagine Planck's 

 assumption is partly justified on the grounds of its 

 simplicity. 



Where, as in this case, a method of investiga- 

 tion does necessarily lead to definite conclusions 

 it is important that these conclusions should be 

 worked out, as they must have a disturbing effect 

 on preconceived theories. 



(2) In 1909 Prof. Max Planck was invited to 

 give a course of eight lectures at the Columbia 

 College, New York, on the present system of 

 theoretical physics. Under the terms of the Ernest 

 Kempton Adams bequest to Columbia University 

 an English translation of these lectures has now 

 been published, drawn up by Prof. A. P. Wills. 

 It will be seen that the date of these lectures is 

 anterior to Planck's enunciation of his quantum 

 hypothesis, which thus forms no part of their 

 contents. 



It is no easy task to give a simple and compre- 

 hensive account of such a vast subject in eight 

 lectures, but Prof. Planck's exposition is remark- 

 able for its conciseness, lucidity, and comprehen- 

 siveness. As a general survey of the subject the 

 ground covered is best indicated by the titles of 

 the lectures, namely, "Reversibility and Irreversi- 

 bility," "Thermodynamic States of Equilibrium," 

 "The Atomic Theory of Matter." "Equation of 

 State for a Monatomic Gas," "Heat Radiation, 

 Electrodynamic Theory," " Heat Radiation, 

 Statistical Theory," "General Dynamics, Prin- 

 ciple of Least Action," and "Principle of Rela- 

 tivity." We may take the last lecture as a good 

 example of the general character of the book. 



Starting with the ordinary notions regarding 

 relative motion of Galileo and Newton, the author 

 first refers to Hertz's theory, and then follows a 

 description of the difficulties Introduced by 

 Fizeau's and Michelson and Morley's experiments, 

 both of which lead to the belief that the relative 

 velocity of light Is independent of the relative 

 velocity of the ether. The author then shows how 

 NO. 2427, VOL. 97] 



these difficulties can be reconciled by the introduc- 

 tion of a new system of space and time co-ordin- 

 ates for moving bodies which will bring the pheno- 

 mena attributed to the ether into accordance with 

 the conventional dynamics of material bodies. 



The book is one which might with advantage be 

 placed In the hands of a candidate for Honours 

 in physics in one of our universities. To read it 

 cannot fail to be of assistance to a student who 

 has to cover a large amount of work In a limited 

 time. G. H. B. 



A CRETACEOUS FLORA. 

 Catalogue of the Mesozoic Plants in the British 

 Museum (Natural History). The Cretaceous 

 Flora. Part ii.. Lower Greensand (Aptian) 

 Plants of Britain. By Dr. Marie C. Stopes. 

 Pp. xxxvi + 360 + xxxii plates. (London: 

 British Museum (Natural History) ; Longmans, 

 Green and Co., and others, 1915.) Price 21s. 



THE Cretaceous Flora, part ii., is devoted to 

 the Lower Greensand (Aptian) flora of 

 Britain. Several species have previously been 

 recorded, but hitherto no general account of the 

 flora as a whole has been written. The most im- 

 portant part of the book Is that which deals with 

 new species of Gymnosperms. Twenty-seven 

 Conifers are described, for the most part repre- 

 sented by cones or petrified wood, nine Cycado- 

 phyta, five Anglosperms, and two Ferns. The 

 Introduction includes some Interesting observa-j 

 tions on climate, a summary of previous work, 

 and remarks on the geological position of thei 

 plant-bearing beds. The descriptions are care-j 

 fully compiled, and the work of other authors; 

 receives frank criticism. A helpful summary is, 

 given of current views on the diagnostic value of 

 different anatomical features in the Identlficatlor 

 of Coniferous wood. The wisdom of employing 

 the generic name Podocarpoxylon for specimen' 

 which cannot as a rule be assigned with certaint^ 

 to the Podocarplneae Is questionable ; but Di 

 Stopes has, on the whole, adopted a judicial atti- 

 tude with regard to the taxonomic value O! 

 anatomical characters. 



One of the most remarkable types is that fa 

 which the new generic name Colymbetes Is prfll 

 posed ; the type-specimen consists of a piece 

 well-preserved wood enclosing a large pith sur 

 rounded by a broad perlmedullary zone ; next t 

 this Is a ring of bundles of vertical trachelds, sue 

 ceeded by . a series of concentric cylinders c 

 secondary wood, composed alternately of vertic;; 

 and horizontal elements. It Is believed that th 

 alternate cylinders are the products of a slngli 

 cambium, which, "for some reason unknowi; 

 turned at right-angles periodically." 



Some new facts are given with regard to Bei 

 nettites Gibsonianus and other Cycadean plant i 

 and a few new types are described. It is suj, 

 gested that the formation of more than or 

 cylinder of secondary wood may be accepted as , 

 distinguishing feature of certain Cycadean stenj 

 referred to Cycadeoldea, the wood of Bennettit' 

 ' being the product of a single cambium. The co 



