December lo, 1908] 



NA TURE 



159 



LETTERS TO THE EDITOR. 



[The Editor does tiot hold himself responsible for opiniotis 

 expressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Students' Physical Laboratories. 

 I A.\i truly sorry that the obituary notice published in 

 Nature two weelcs ago should seem to Sir O. Lodge to 

 minimise the work ol Prof. Carey Foster and others. I 

 feel sure that nobody can value Prof. Foster's work more 

 than 1 do, but he had neither the money nor the other 

 opportunities that Prof. Ayrton had in Japan. I admit a 

 little overstrain in the statement that at the time when 

 he created his Japanese laboratory " there were not half 

 a dozen people in Great Britain who had experimented in 

 electricity." I ought to have said that there were only a 

 few workers in electricity. I had in my mind that before 

 starting for Japan early in June, 1875, I had the curiosity 

 to count the number of electrical papers published before 

 the Royal Society, and now printed in vols. xxii. and xxiii. 

 of the Proceedings. I had no linowledge of meetings after 

 May 13, 1875, as 1 lived in Glasgow., At the forty-one 

 consecutive meetings from December 11, 1873, to May 13, 

 1S75, there were in all only five papers read having a bear- 

 ing on electricity. These were two by Dr. Gore, one by 

 Prof. .Adams, one by Messrs. de la Rue, Hugo Miiller 

 and Spottiswoode, and one by Prof. Balfour Stewart. I 

 was on my way to Japan when my own first published 

 electrical investigation was described at the Royal Society 

 on June 10, 1875. 



I do not think that with a record like this it is worth 

 while to cavil at my statement, for it is to be remembered 

 that Royal Society papers, not electrical, were numerous. 

 For example, at the meeting on June 18, 1874, there were 

 twenty-eight papers, and on June 11 there were eight 

 papers, and not one of these thirty-six papers had any- 

 thing to do with electricity. I have not referred to a few 

 papers during the year on terrestrial magnetism. It was 

 with impressions due to this knowledge that I first saw 

 the Japanese laboratory, and when I wrote the obituary 

 notice my old feeling of overpowering admiration had come 

 back. 



In writing about Finsbury I ought perhaps to have 

 expressed myself more clearly. Sir Oliver Lodge mis- 

 understands me. Everybody knows that at King's and 

 University Colleges, and at many other colleges, students 

 were allowed to work in laboratories, and I can imagine 

 that it was a great privilege to Sir Oliver to work under 

 Carey Foster, whose record as a pioneer, as a teacher, and 

 as a writer is so high that it is almost an impertinence 

 in me to refer to it. Volunteer boys did excellent work 

 in my own laboratory at Clifton College in 1871, just as 

 Kelvin's students had worked much earlier in Glasgow; 

 but I think I was right in saying that in all such cases 

 the students were few in number, and that they were 

 volunteers. My point was that all the students at Fins- 

 bury had much laboratory work, and they were made to 

 think that laboratory work was much more educational 

 than attendance at lectures. I still think that the reform 

 effected at Finsbury was exceedingly great, and that it 

 was of quite a new kind, for it was not only in the nature 

 of the laboratory work, but in its combination with many 

 other kinds of work, that the reform consisted. I cannot 

 hope to carry Sir Oliver with me, for it is quite evident 

 that he knows of Finsbury only at second or third hand. 

 He seems to think that there were only evening classes. 

 It is true that many of the evening students were of the 

 artisan class ; but the day classes were of much greater 

 importance than the evening classes, and students of the 

 ages of sixteen to eighteen coming from secondary schools 

 will not fit into his description. 



I am glad to think that Sir Oliver approves of that 

 small part of the Finsbury work of which he has heard. 

 No doubt much may be said for and against some of the 

 Finsbury methods, but I do not care to continue a dis- 

 cussion founded on an obituary notice. I know of no 

 obituary notice which might not be the subject of con- 

 troversy. John Perry. 

 nocember ^. 



NO. 2041, VOL. 79] 



A Model Atom, 



The following attempt to construct a kinematics of an 

 atom may prove of interest to readers of Nature. 



Let a sphere of a certain radius (depending on the given 

 circumstances) be described round each charged particle 

 as centre, and let the radii of these spheres be such that 

 some of the spheres are in contact. The spheres may be 

 called spheres of interference, and the points of contact 

 nodes. The spheres associated with two oppositely charged 

 particles may be supposed to touch internally, and those 

 associated with two particles carrying similar charges may 

 be supposed to touch externally. 



A model atom may now be built up of spheres touching 

 one another in this way. We shall suppose that there is 

 one sphere surrounding all the others, which we shall call 

 the atomic sphere. Within this sphere there may be other 

 spheres which completely surround a number of others. 

 Such groups will be called subatoms. 



As the electrons within the atom move about we shall 

 suppose that in general their spheres of interference adjust 

 themselves so that the contacts are preserved ; such a 

 motion may be called a steady motion, and may be 

 obtained by applying a continuous succession of conformal 

 transformations to a given configuration of the spheres or 

 set of spheres. 



When an atom is in a normal state we shall suppose 

 that the outer shell contains either a ring of electrons the 

 spheres of interference of which touch one another in 

 succession, and also touch two other spheres, one internally 

 and the other externally, or a system of electrons at the 

 corners of a polyhedron, the spheres being now arranged 

 so that each one touches all its neighbours and two other 

 spheres as before. 



If the two extra spheres are kept fixed the electrons can 

 move round an ellipse, so that the contacts of the spheres 

 are preserved, the radius of a sphere being at any time 

 proportional to the distance of its centre from the radical 

 plane of the two fixed spheres (Steiner's porism). If now 

 the mass to be associated with a given electron or sphere 

 of interference belonging to the ring is inversely propor- 

 tional to the square of the radius of the sphere, the total 

 mass, kinetic energy, and position of the centre of mass 

 will remain invariable so long as all the contacts are 

 preserved. 



When an atom is ionised we may suppose that there is 

 one sphere missing from the ring if the charge be positive, 

 and an extra sphere in contact with two spheres of the 

 ring, but not belonging to the ring, if the charge be 

 negative. If the number of degrees of freedom is calcu- 

 lated by allowing three for each electron and subtracting 

 one for each contact or other geometrical condition, there 

 will be a gain of one degree of freedom for each additional 

 charge, whether it arises from the gain or loss of an 

 electron. 



We may suppose that a line spectrum is emitted when 

 a given arrangement of nodes or geometrical conditions is 

 preserved, and a continuous spectrum when the geometrical 

 conditions are violated. 



The group of infinitesimal conformal transformations 

 seems the natural one for describing the kinematics of a 

 system within a sphere ; it may be built up from successive 

 inversions with regard to spheres, just as the group of 

 displacements of a rigid body may be built up from 

 successive reflexions in different planes ; it should be 

 noticed, however, that an even number of inversions are 

 required to produce an infinitesimal change. 



An inversion does not alter the type of contact of two 

 spheres when the centre of inversion is external to both, 

 but when it lies in the space between two spheres the type of 

 contact changes, and the spheres become external to one 

 another. This may be regarded as a kinematical descrip- 

 tion of a radio-active process, for a subatom may be 

 thus brought outside the atomic sphere by a continuous 

 succession of changes. According to _ this view, an atom 

 would break up whenever one of the centres of inversion 

 happened to lie within the atomic sphere. 



We suppose that in general the arrangement of spheres 

 within the atom is not symmetrical ; if, for instance, the 

 atom forms part of a molecule, the field of force is not 

 symmetrical, and there seems no reason why the arrange- 

 ment of the electrons should be so. 



