July 30, 1891] 



NATURE 



299 



the energy given out by the batteries over that spent in 

 heat. Thus 



and this is the work done in virtue of changes of the 

 currents. This quantity must be a perfect differential, 

 since its integral vanishes for a closed cycle of changes. 

 The condition which must hold for this enables the 

 values of A, B, C to be identified with - L,, - M, -Lg. 



Maxwell's introduction of Lagrange's dynamical method 

 into electro-magnetism is, as has been already stated, 

 regarded by Pomcard as of great importance, and as he 

 says " nous touchons ici h la vraie pensde de Maxwell." 

 After finding by this method the inductive electromotive 

 forces, and the electro-magnetic forces, he proceeds to 

 discuss Maxwell's theorems of the electro-magnetic field, 

 and their crowning generalization, the electro-magnetic 

 theory of light. Except here and there, the treatment 

 differs only in points of detail from that of Maxwell. 



With regard to the equations of currents, 



« = CP + ^^/. 

 47r dt' 



&c., &c., 



a difficulty is pointed out as to the specific inductive 

 capacity of a conducting substance. For such a sub- 

 stance the first term must preponderate, and so K must 

 be small ; whereas K is generally regarded as very great 

 in the case of a conductor. It is worth noticing that this 

 is really only a conventional means of explaining the 

 impossibility of charging a condenser the space between 

 the plates of which is filled with conducting substance ; 

 the true explanation is, no doubt, very different. 



The discussion of the experimental verifications of the 

 electro-magnetic theory of light contains references to 

 several lately-established experimental facts (apart from 

 Hertz's experiments, which are reserved for special 

 treatment) which bear on the theory. For example, it 

 has been shown by Curie that dielectrics, when tabulated 

 in the order of increasing conductivity, are on the whole 

 arranged (as obviously they should be) in the order of 

 diminishing diathermancy. Further, ebonite, which is 

 opaque to light, is very permeable to dark radiations of 

 longer period, which agrees with its high transparency to 

 electrical waves. 



Again, it is remarked that the results of the electro- 

 magnetic theory with regard to reflections from the sur- 

 face of glass and of metals lend a general support to the 

 theory, while the disagreement in the values of the 

 numerical constants as regards the want of magnetic 

 permeability is referred to the frequency of the vibrations 

 and the fact that the magnetization of the medium is not 

 instantaneously produced. 



A marked feature of M. Poincar^'s treatise is the 

 chapter on rotatory polarization, in which he discusses 

 the phenomena of rotation of the plane of polarized light 

 by the action of a magnetic field. Although the essential 

 difference between this effect and the apparently similar 

 action of quartz, sugar solutions, &c., is pointed out, 

 the author does not appear to lay stress on it as 

 throwing light on the difference between their causes. 

 For example, after giving Airy's differential equations, for 

 the propagation of the two rectangular component displace- 

 ments, I, T], of a circularly polarized wave travelling along 

 the axis of 2, in the form 





dzW 



from which a formula for the rotation of the plane of 

 polarization of plane-polarized light in a magnetic field 



NO- I 135, VOL. 44] 



can be obtained, which agrees with experiment ; and after 

 comparing the results of these equations with those of 

 other proposed equations, he says : — 



" Mais si le concordance de la formuleavec rexperience 

 justifie introduction des deriv<fes 4- d^rj/dz^dt, — d'^^/dz'di 

 dans les secondes membres des Equations du mouvement 

 d'une molecule d'^ther, aucune consideration thdorique 

 ne preside au choix de ces ddrivdes h I'exclusion des 

 autres ; on ne possedait done pas encore de thdorie de la 

 polarisation rotatoire magndtique." 



This certainly seems rather too strong a statement in 

 the face of Thomson's dynamical theory outlined in his 

 " Electrostatics and Magnetism," and further elaborated 

 in Maxwell's treatise. 



Thomson's views on this subject are of the most 

 fundamental importance, as they point to motion of, or 

 in, the medium occupying the magnetic field as the 

 cause of the magneto-optic effect discovered by Faraday, 

 and to a certain structure of the substance as producing 

 the phenomena shown by quartz, syrup, &c. One of the 

 most interesting passages of his lectures on molecular 

 dynamics, delivered at Baltimore in 1885, is that in which 

 he accounts for the observed results by the presence of 

 rotatmg particles, "gyrostatic molecules," in the medium. 



It is obviously suggested by the gyrostatic investigation 

 that it ought to be possible to explain the magneto-optic 

 rotation in the electro-magnetic theory of light as a con- 

 sequence of the presence of small magnets embedded in 

 the vibrating medium with their axes in the direction of 

 the ray ; and therefore producing a component of mag- 

 netization in that direction. It is stated by M. Poincard 

 that a theory of this kind has been proposed by M. Potier, 

 and published in the Comptes Rendus. The theory itself 

 is not given, but the differential equations obtained are 

 quoted, and are of the required form, and lead to the 

 known experimental result.^ 



Maxwell's molecular vortices theory is, however, given, 

 and certain difficulties which it involves discussed. The 

 theoretical results of Hall's experiment are also given in 

 this connection, and Kerr's experiment proving the pro- 

 duction of elliptic polarization by the reflection of plane- 

 polarized light from the pole of a magnet is cited, but 

 without any statement of the theory of the effect which 

 has been worked out, principally by Fitzgerald. With 

 regard to the explanation of the Hall effect by strain of 

 the conducting film produced by the magnetic field, it 

 has always seemed to me that it ought to be possible 

 with proper appliances to decide the question, by experi- 

 mentmg with a sufficiently powerful and uniform magnetic 

 field. 



The work, it ought to be stated, concludes with an 

 interesting chapter by the editor, M. Elondin, on experi- 

 mental verifications of the theories of Maxwell. This 

 comprises the chief determinations of specific inductive 

 capacity, Kerr's classical researches, and lastly, the 

 interesting investigation made by M. Rontgen of the 

 electro-magnetic action of currents of displacement. 



Of Prof. Poincard's second treatise on the experiments 

 of Hertz, &c., I hope shortly to give an account as a 

 sequel to the present article. A. Gray. 



THE ORIGIN OF THE FLORA OF GREENLAND, 



HOW the present flora of Greenland originated, is 

 a question of great interest to British botanists and 

 geologists, for the answer will probably help to solve the 

 difficult problem. What was the origin of the recent flora 

 of Britain .? The flora of Greenland is so poor in species 

 and has been so well studied that its relationship to the 

 floras of Europe and America ought not to admit of much 



Poincard's reference has suggested to me a mode of investigating 

 ion of these magnets on the electro-magnetic theory. This is 



• M. 

 the action ot these magnets 



discussed in a separate article, which contains for the sake of comparison an 

 account of the gyrostatic theory. 



