170 



NA TURE 



[June 22, 1905 



and retarders on amylohydrolysis, there is no mention 

 of Ford's recent work. 



Cytase is stated not to occur in resting seeds, but 

 it is present in small amounts in some. 



In giving directions for tiie preparation of 

 Schweizer's solvent for cellulose, the best method, 

 that of dissolving metallic copper in ammonia through 

 which a current of air is passed, is not mentioned. 



The function of a critic is to criticise; but he is 

 human, and hence prone to eulogise — or blame. In 

 this case the reviewer can only yield praise. The 

 work fills a void that botanical physiologists have 

 long felt. The wealth of material dealt with is sur- 

 prising, and the expenditure of labour must have 

 been enormous. There is very little evidence of par- 

 tiality, whether national or of other kind. The com- 

 pass of the work is wide, and it is thoroughly up to 

 date. 



The reviewer was especially pleased with the 

 general part. For the first time in a botanical work 

 adequate stress is put on the importance of colloids 

 as such, and on the necessity of knowledge of their 

 nature for progress in physiology; and for the first 

 time the principles of general chemistry are given the 

 position due to them in a work of 'this class. To 

 Prof. Czapek for having done this all botanical 

 physiologists must remain indebted. 



In the opinion of the reviewer the value of the work 

 would have been enhanced by incorporation in it of 

 curves illustrating the various processes described 

 and by citation of mathematical expressions wherever 

 they have been proved or found to be applicable. 



If the work should run through subsequent editions 

 as IS most likely, it would certainly be best for 

 different authors to be delegated for "various parts 

 smce with the rapid accumulation of material it will 

 soon be impossible for a single author to deal 

 adequately with a work of such dimensions. 



F. ESCOMBE. 



THE ELECTROMAGNETIC THEORY OF 

 INERTIA. 

 (i) Mathematische Einfiihrung in die Elektronen- 

 theorie. By Dr. A. H. Bucherer. Pp. j.g 

 (Leipzig and Berlin: Teubner, 1904.) Price 7.20 

 marks. 



(2) Experimentelle Elektrizitatslehre. By Dr. H. 

 Starke. Pp. xiv + 422. (Leipzig and Berlin: 

 Teubner, 1904.) Price 6 marks. 



(3) Leitjaden der Physik fur die oberen Klassen der 

 Realanstalten. By Dr. F. Bremer. Pp. viii + 294. 

 (Leipzig and Berlin : Teubner, 1904.) Price 3.20 

 marks. 



(i) "T^HE property of matter which has always been 

 -L regarded as most fundamental is "inertia." 

 This property is adopted as the measure of quantity 

 of matter in dynamics, and the nearest approach to 

 a complete explanation of a physical phenomenon is 

 generally supposed to have been reached when the 

 phenomenon has been shown to be due to the motion 

 of particles possessing inertia. We may say, in fact, 

 that the tendency of nineteenth century physics was 

 NO. i860, VOL. 72] 



to give a purely dynamical explanation of everything. 

 A striking example of this tendency is Maxwell's 

 dynamical theory of the electromagnetic field. 



In 1881 it was shown by Prof. J. J. Thomson that 

 a particle charged with electricity possesses some 

 inertia due to its electric charge in addition to the 

 ordinary inertia of the particle. As the result of 

 Kaufmann's measurements, we now know that all 

 the inertia of an electron is of this electromagnetic 

 kind. It is now further suggested that all matter is 

 composed of electrons, so that all inertia is electro- 

 magnetic. Density, according to this view, is simply 

 number of electrons per unit volume. Electro- 

 magnetic inertia, that is, all inertia, is due to the 

 energy of the magnetic field produced by the moving 

 charge of electricity. The energy of this magnetic 

 field resides in the ether. .According to Maxwell's 

 dynamical theory, the electromagnetic energy of the 

 ether is due to motion of parts of the ether, these 

 parts possessing inertia. But the only kind of inertia 

 which we really know is the inertia of matter, which 

 IS due to the electromagnetic action of the electrons 

 of which matter is made up. If inertia is due to 

 electrons, then if we ascribe to parts of the ether the 

 property of inertia we ought to say that the ether 

 contains so many electrons per unit volume. But the 

 free ether is not supposed to contain any electrons; 

 in fact, if we explain inertia by the energy of the 

 magnetic fields produced by moving charges, then 

 evidently to explain this energy by inertia in the ether 

 is merely to argue in a circle. The position is, then, 

 that inertia is explained in terms of the electro- 

 magnetic field, and that now some explanation of this 

 field is required not involving inertia at all. So 

 far, no such explanation has been offered. Larmor 

 has suggested that the ether has an enormous density 

 or inertia per unit volume, and that it moves along 

 the lines of magnetic force. This explanation must 

 evidently be abandoned if matter is regarded as made 

 up of electrons having only electromagnetic energy, 

 or else we must say that the ether has a sort of 

 inertia, not the same as the inertia of matter, but 

 like the kind of inertia matter was supposed to have 

 before the electromagnetic theory of inertia was put 

 forward. 



To say this, however, is merely to ascribe to the 

 ether a property the definition o'f which is that it 

 explains what it is required to explain ; it is, in fact, 

 merely the old process of " explaining " a thing by 

 inventing a name for its explanation. The properties 

 of the ether, then, expressed by Maxwell's system of 

 equations are at present without any explanation, but 

 they have taken the place of inertia as the funda- 

 mental thing in terms of which phenomena are to be 

 e.xplained. In these circumstances the appearance of 

 Dr. Bucherer's small volume on " Elektronentheorie " 

 is exceedingly opportune. It contains a concise and 

 readable account of Lorentz's splendid theory and of 

 the electromagnetic theory of inertia. The introduc- 

 tion gives a short account of the properties of kathode 

 rays and radium radiation, which, of course, are the 

 properties of electrons on which the electronentheorie 

 is based. Chapters i. and ii. contain a short account 

 of Lorentz's theory for bodies at rest, and chapters 



