lO 



NATURE 



INOZK 



The Queen's Jubilee Prize Essay of the Royal 

 Botanic Society of London. 



In your issue of October i8 appears (p. 594) a review of the 

 essay for which I was awarded the medal of the Royal Botanic 

 Society, in which the writer makes a great point of my omitting 

 all reference to drugs. He does not state, for the information 

 of your readers, that the prize was offered for the best (not 

 necessarily complete) essay on the "Vegetable Substances in- 

 troduced into Britain for use in the Arts, Manufactures, Food, 

 and Domestic Economy during the Reign of Her Majesty Queen 

 Victoria." It is not necessary that one should be either "a 

 member of the medical profession" or have "a wholesome 

 dread of drugs " to know that drugs used as medicines could not 

 with any fitness be introduced into this essay ; indeed, inquiry 

 from the Secretary elicited the fact that they had been purposely 

 excluded. 



Had your reviewer read the essay with any care, he would 

 have observed that I quote Dr. Forbes Watson to the effect that 

 China grass and rhea fibre are products of the same plant , but 

 ]ircpared in different ways ; while an unprejudiced reviewer 

 would have mentioned that the quotation having reference to 

 Phor?nuim tenax is preceded in the essay by the words, " In one 

 of the authorities consulted it is stated that New Zealand flax . . . 

 was introduced into England about 1840 ; but the author has 

 found a reference to an unsatisfactory attempt to weave it at 

 Knaresborough at a much earlier period than this, and that it 

 had been experimented upon in the Portsmouth Dockyard about 

 1819, the ropes made from it being satisfactory." 



It was evident that the judges considered that "gun-cotton 

 and its derivatives" are "direct products of the vegetable 

 kingdom," or they would not have printed this chapter of the 

 essay. 



The limited time allowed for the preparation of the essay 

 (about four months), and the inability of the author to avail 

 himself of any collection of economic botany and of many of the 

 most recent books on the subject, naturally led to many defi- 

 ciencies in the list of substances mentioned, and of this no one 

 was more conscious than the author hiaiself ; and all he claims 

 for his essay is that, in the opinion of the judges (one of whom 

 was Prof. Bentley), it was the best of the half-dozen sent in in 

 competition. John W. Ellis. 



3 Brougham Terrace, Liverpool, October 23. 



I HAVE but few remarls to make in answer to Mr. Ellis's 

 letter. First, I cannot follow his reasoning that completeness 

 should not in some measure count as a test of quality, nor can 

 I see anything in the preamble of the offer of the prize to exclude 

 drugs. Mr. Ellis is justified, however, in having done so by 

 receiving direct information from the Secretary to that effect. 



On the subject of China grass and rhea, the author, in his 

 essay, distinguishes them under separate heads, describing the 

 first rightly as the produce of Bcelwieria nivea, and the second 

 as " the produce ol^ the East Indian Bcehmeria \Urtica) tenacis- 

 sinia, a congener of the species producing China grass." It is 

 after this authoritative statement that he refers. to Dr. Forbes 

 Watson's opinion. 



Regarding New Zealand flax {Fhorvihim tenax), Mr. Ellis, 

 in his essay, follows up the quotation given in his letter by the 

 following paragraph : " Not having been introduced during the 

 period to which this essay refers, any further mention of this 

 interesting fibre — for which it has frequently been attempted to 

 find a place in the British market — is unnecessary ; " thus justi- 

 fying my remarks on this head. 



I leave it to anyone who has read Mr. Ellis's chapter on 

 "Gun-cotton and its Derivatives," to say whether they are 

 direct products of the vegetable kingdom. 



I'he latter part of Mr. Ellis's letter, I think, supports the 

 truth of my review generally. The Reviewer. 



October 27. 



MODERN VIEWS OF ELECTRICITY? 

 Part IV.— Radiation. 



XII. 



'\'\7E must now mention one or two phenomena which 



*^* depend entirely upon a modification of ether by 



the neighbourhood of matter, and which we have reason 



' Continued from vol. xxxviii. p 592. 



to believe would not occur in free ether at all. These 

 are the optical phenomena of Faraday and Kerr, and the 

 electric phenomenon of Hall. 



Faraday discovered, long before there was any other 

 connection known between electricity and light, that the 

 plane in which lighl-vibrat'ons occur could be rotated by 

 transmitting light throug'a certain magnetized substances 

 along the lines of magnetic force. To make this effect 

 easily manifest, one uses plane-polarized light and trans- 

 mits it through a fair length of magnetized substance, 

 analyzing it after emergence, and showing that, though 

 it remains plane-polarized, the plane has been rotated, 

 possibly through a right angle or more. 



Now, in a general way it is easy to imagine that, inas- 

 much as something of the nature of a rotation is going 

 on in a magnetic field round the lines of force, vibrations 

 travelling into such a field along these lines should be 

 twisted round, corkscrew fashion, and emerge vibrating 

 in a different plane. But when one tries to follow out 

 this process into detail, one finds it not quite so simple 

 a matter. It has no business to be a very simple and 

 obvious consecpience of the existence of a magnetic 

 rotation round the rays of light, else would it occur in- 

 free space, and in the same direction in all media. But the 

 facts are that in free space — that is, in free ether — it does 

 not occur at all, and the direction of rotation is not the 

 same for all media : substances can, in fact, be divided 

 into two groups, according to the way in which given 

 magnetization shall rotate the plane of polarized light 

 passing through them. 



Similarly with the electrostatic optical effect discovered 

 by Dr. Kerr, who showed that plane-polarized light trans- 

 mitted across the lines of force in an electrostatic field 

 could, in certain media, come out elliptically polarized. 

 Now, inasmuch as an electric field is a region of strain, 

 and strain in transparent bodies is well known to make 

 them slightly doubly refracting and able to turn plane- 

 polarized into elliptically-polarized light, it is very easy to 

 imagine such a result in an electric field to be natural 

 and probable. But the explanation is not so simple as 

 that, else it ought to be a large effect, occurring in all 

 sorts of media in the same direction, and likewise in free 

 space. But the facts are that it does not occur at all in 

 free space, and it occurs in different senses in different 

 substances ; so that again they can be grouped into two 

 classes according to the sign of the Kerr effect. 



Thus, then, the rotatory effect of a magnetic field upon 

 light, discovered by Faraday, and the doubly refracting 

 effect of an electrostatic field upon light, discovered by 

 Kerr, agree in this : that they are both small or residual 

 effects, depending on the existence of a dense medium, 

 and both varying in sign according to the nature of the 

 medium. 



The only substance in which the Faraday effect is large 

 is iron, including with iron the other highly magnetic sub- 

 stances. The discovery of the effect in these bodies was 

 likewise made by Kerr. The difiiculty of dealing with them 

 is that they are very opaque, and hence that the merest 

 film of them can be used. The film can be used either 

 by way of transmission or by way of reflection, i^ matters 

 not which, but reflection is perhaps the more convenient. 

 Light reflected from the pole of a magnet has indeed 

 barely penetrated at all into the substan,:e of the iron 

 before being sent back ; still, it has penetrated deep 

 enough to be distinctly rotated by the tremendous mag- 

 netic whirl which it finds there. "All these highly mag- 

 netic substances are metallic conductors, and are therefore 

 very opaque. 



Whether there is any real connection between high 

 magnetic susceptibility and conductivity is more than I 

 can say. But it is quite natural, and indeed necessary, 

 that the greatest portion of light should be reflected on 

 entering a highly magnetic medium, because in such a 

 medium the ethereal density, [>., is so great, and hence the 



