July 23, 1903] 



NA TURE 



269 



tural qualities and representative uses. These will 

 prove of value to both expert and amateur. 



From its title one might be led to suppose that the 

 book was an addition to the literature of strict forest 

 botany, but the preface states that " It is intended for 

 those who are not foresters or botanists, but who use 

 woods or desire a knowledge of their distinguishing 

 properties." The preface further states that " Although 

 great care has been taken to check each fact, errors no 

 doubt exist, although it is not believed that there are 

 important ones." \Ve cannot entirely agree with the 

 author in this. For example, in the introduction we 

 are told that a true wood fibre originates from several 

 cells, " a resin duct is a cell structure or a fibre," " a 

 vessel is a short wide tube joined vertically end to end 

 with others of its kind." 



Inaccuracy and vagueness of expression are to be 

 found elsewhere in the book. For instance, "Euro- 

 peans regard the Ash for ornamental purposes, but 

 Americans value it for wood " is an error that may 

 perhaps be excused in an American writer, but why 

 should the leaves of Eucalyptus be described thus? — 

 " Those of young blue gums are bright blue, oval and 

 stalkless, while leaves of older trees have stems (sic), 

 are dark green and sickle-shaped." 



Attention is further directed in the preface to the fact 

 that " Allusions to trees, historical and other references, 

 aside from those directly regarding woods, are made 

 for completeness and in order to mark, distinguish, or 

 separate the species." The author fails to realise this 

 object. The distinguishing characters given are far 

 too vague and general to be of any practical value. 



On the whole the book contains much useful informa- 

 tion and statistics regarding the various species of 

 wood, both broad-leaved and coniferous. It would 

 have been much better, however, had the author con- 

 fined himself to the treatment of this aspect of the sub- 

 ject alone, leaving 6ut all botanical and other technical 

 matter. 

 Lehrbuch der Mikrophoto graphic. By Dr. Carl 



Kaiserling. Pp. viii + 179. (Berlin : Gustav 



Schmidt, n.d.) Price 4 marks. 

 Although there are several well-known treatises on 

 this subject, it is doubtful whether any exceed in 

 thoroughness the one now under notice. The essential 

 conditions for the production of photomicrographs of 

 the highest class are carefully described, and each part 

 of the process is treated fully. 



There is no more important point than the illumin- 

 ation of the object itself, and both the source of light 

 and its colour should be selected to bring out the 

 desired points in the resulting photographs. 



This part of the subject is generally treated all too 

 briefly, but in the present instance its importance is 

 evidently recognised. The various ways of making 

 light filters and their use with coloured preparations 

 are described. The method of arriving at the proper 

 filter to use with a given preparation is stated to be 

 by determining the absorption spectrum of the dye 

 used for staining, by aid of a hand spectroscope, and 

 then adapting the light filter to give the result desired. 

 This is undoubtedly the only scientific method of using 

 colour screens in photomicrography, and one which 

 we have adopted with success for some time past. 



The various types of apparatus by the leading 

 makers are fully described, prominence being 

 naturally given to continental firms. Instructions as 

 to :he use of substage apparatus, methods of centring, 

 choice of objectives, and the combination of micro- 

 scope and camera are included, while it is satis-. 

 factory to note that no space is unnecessarily wasted 

 over purely photographic processes. Altogether the 

 book may be recommended to photomicrographers as 

 one of the best yet published. J. E. B. 



NO. 1760, VOL. 68] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond ivnh the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Source of Radium Energy. 



The novel and unforeseen property of radium of pro- 

 ducing energy, which purely kinetic theories, in opposition 

 to the notion of inherent force as a transcendental element, 

 do not seem able to explain, is perhaps destined to give 

 a fresh impetus to discussion from the two distinct points 

 of view. It is meanwhile to be noted with regard to this, 

 that the notion of force acting at a distance from point 

 to point, being equal and reciprocal between the various 

 material points, does not appear to be any better met by 

 the manifestation of the unfailing energy of radium than 

 the simple movements of the kinetic theory. This re- 

 mark justifies attention being directed to a view of the 

 natural physical forces presented by the present writer more 

 than ten years ago (see Lagrange's " Study of the System 

 of Physical Forces," forming vol. xlviii. of the Memoirs 

 of the Royal Academy of Sciences of Belgium). It is there 

 shown that forces exist of such a nature that static equil- 

 ibrium is impossible, on the impact of bodies of different 

 composition, at their surfaces of contact. They are forces 

 making a body, after the example of radium, emit rays 

 unceasingly without apparent loss of substance. A . force 

 of repulsion is referred to here, emanating from the surface, 

 and not from the centre of the mass of atoms, acting on 

 opposed surfaces, and the varying intensity of which is 

 nothing else than what is known to science as absolute 

 temperature. That repulsive force, acting in the inverse 

 ratio of the volume of matter (or of the cube of the dis- 

 tance), just as Newtonian gravitation acts in the inverse 

 ratio of the surface (or as the square of the distance), takes 

 its immediate development, and to some extent visible 

 shape, in Mariotte's law of the relation of pressure to 

 volume in gases. The memoir establishes the existence of 

 a continuous interatomic medium of transcendental quali- 

 ties not yet understood, conveying the effect of a force 

 acting at the surface of atoms, and the real seat of 

 luminous and electromagnetic wave motion, according to 

 the views to which clearly Lord Kelvin has of late returned. 

 The view now presented is entirely deduced from analysis 

 of the actual facts, worked out at length, and justified by 

 the memoir, and new so far as the case of the impossibility 

 of an equilibrium due to the surface force of repulsion, 

 which gives rise to an exhaustless emission of energy. 

 The reflecting attention of physicists may therefore be 

 legitimately directed to the subject, because it seem» 

 certain that the new properties which radium manifests 

 are not explainable by the kinetic hypothesis, but, on the 

 contrary, are of a nature henceforward to modify consider- 

 ably the speculations of modern physics. 



Brussels, July 14. Ch. Lagrange. 



A New Case of Phosphorescence induced by Radiumi 

 Bromide. 



It is known that salt (NaCl) at a temperature of 200° C. 

 is phosphorescent {vide Phipson on " Phosphorescence," 

 p. 20) ; during a course of experiments in June last I found 

 that radium bromide induces phosphorescence at ordinary 

 temperatures. The following is a convenient way of 

 observing the phenomenon. Fill a wooden match-box with 

 table salt removed from the inner portion of a block ; press 

 the radium bromide tube into the yielding mass and just 

 barely cover it with the substance. If it be now put on one 

 side for a few hours, say into one of the compartments of 

 a chest of drawers, on opening the box in the dark all 

 round the tube will be found to phosphoresce with a white 

 light, but, unlike zinc blende and barium platinocyanide,. 

 the salt continues visibly to phosphoresce after removal of 

 the radium bromide. The portions of salt round the tube 

 are turned of a faint buff or ochrey tint. The image of 

 the visible portion round and where the radium broniide 

 tube has lain is impressed on a photographic plate in thirty 



