124 



NATURE 



[June 9, 1904 



his preliminary qualitative study of the elements, is a 

 knowledge of a few typical processes carefully selected 

 to illustrate the principles of quantitative analysis. 



It is for this reason that the exercises which form 

 part ii. of this volume have much to recommend them. 

 Those who are accustomed to the old system, which 

 probably originated in Berzelius's laboratory, of 

 analysing a few inorganic salts and then a series of 

 minerals, will probably be startled at finding such an 

 incongruous collection as lead carbonate, sodium 

 chloride, coffee, cast iron, vinegar, &c., following one 

 another. Yet when one considers how few students, 

 after taking a substantial course in practical chemistry, 

 could suggest a means for distinguishing between lard 

 and vaseline, or benzene and petroleum, except by their 

 smell, it must be admitted that a practical acquaintance 

 ■ with common materials in the form of analysis has 

 many advantages. 



Perhaps the best way of estimating the value of a 

 book is to ask oneself whether one would care to possess 

 it For those who are interested in any kind of general 

 or technical analysis, this question may be safely 

 answered in the affirmative. J. B. C. 



Catalogue of the Mesozoic Plants in the Department of 

 Geology, British Museum (Natural History). The 

 Jurassic Flora. II. Liassic and Oolitic Floras of 

 England (excluding the Inferior Oolite Plants of the 

 Yorkshire Coast). By A. C. Seward, F.R.S. Pp. 

 XV+192. Plates i-xiii. (London: Printed by Order 

 of the Trustees of the British Museum, 1904.) 

 The concluding part of Mr. Seward's catalogue of the 

 Jurassic floras makes the catalogue a nearly complete 

 treatise on the known fossil remains of plants from the 

 Trias, Rhaetic, Lias and Oolite of England. In the 

 present volume are included plants from Jurassic rocks 

 in various parts of England, together with a few from 

 the Coralline Oolite and Liassic strata of East York- 

 shire. A short account is also given of such Triassic 

 and Rhaetic plants as are represented in the British 

 Museum. 



Die Kathodenstrahlen. By G. C. Schmidt. Pp. 



vi+i2o. (Brunswick: Vieweg und Sohn, 1904.) 



Price 3 marks. 

 This book contains a concise and complete account of 

 the properties of kathode rays, presented in an ele- 

 mentary way, together with as much information on 

 the electric discharge and allied phenomena as is neces- 

 sary for their comprehension. 



The information is brought well up to date and 

 references to the original papers are given. The book 

 should prove of use to those wishing for a connected 

 account of the subject in a readable form devoid of 

 mathematical analysis. The fifth chapter contains a 

 short but interesting account of the historical develop- 

 ment of the theory of the electric discharge and kathode 

 rays. H. A. W. 



An Introduction to Metal Working. By C. J. Pearson. 



Pp. 106. (London : Murray, 1904.) Price 2S. 

 This little book is evidently intended to describe the 

 ordinary tools used in the working of metals, although 

 the author does not tell us so. These descriptions are 

 of an elementary nature, and well suited for young 

 students in the early stage. The book is fully illus- 

 trated with woodcuts, as well as with some very excel- 

 lent photographs of operations, the latter being a 

 distinct novelty in a technical book. The author uses 

 simple language, he knows what he is writing about, 

 and we feel sure that his little work will be much 

 appreciated by junior students, apprentices and others, 

 to whom we strongly recommend it. 



NO. 1806, VOL. 70] 



LETTERS TO THE EDITOR. 



[The Editor docs not hold himself responsible for opinions 

 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.] 



A Dynamical System illustrating the Spectrum Lines. 



In Nature of March 10, which I received yesterday, Prof. 

 Schott remarks that the dynamical system illustrating the 

 spectrum lines, which I communicated to Nature of 

 February 25, is unstable. This evidently rests on a mis- 

 understanding, because the system which I have discussed 

 is not identical with that which was investigated by Prof. 

 Schott. I believe that the statement, " such an ideal atom 

 will not be contradictory to the results of recent experi- 

 ments," led Prof. Schott to suppose that the system which 

 I considered must be electrically neutral. Those who have 

 read Maxwell's paper on the stability of Saturn's rings 

 will admit, without Prof. Schott's comment, the futility of 

 discussing an electrically neutral system with a central 

 positive charge and a ring of negative electrons as 

 satellites. 



The system which I have investigated is not electrically 

 neutral, but the central charge is supposed to be very large 

 compared with the negative charges in the ring. Since q 

 (using Prof. Schott's notation) is great compared with J or 

 K, the ring is generally stable ; the proof for stability can 

 be given in a more general manner. 



To prevent furtlier misunderstanding, it will be necessary 

 to explain how I was led to investigate such a system 

 instead of an electrically neutral one. .-\s the principal aim 

 of the investigation was to discuss the small oscillations 

 which will illustrate the regularity in line and band spectra, 

 it was necessary to have a hypothetical atom, which will 

 have some resemblance to an actual one, and the motion 

 of which can be treated by means of simple mathematical 

 analysis. Recent investigations show that a chemical atom 

 is associated with numerous electrons, which in the lightest 

 element amount to several hundred, while in heavy atoms 

 the number may reach a hundred thousand. We have 

 reason to believe that these electrons are not crowded 

 together in spite of their large number. In making 

 abstraction as to the hypothetical atom, we may conveniently 

 assume the central positive charge to form a neutral system 

 with all the negative electrons, but only a small fraction 

 of the latter to be found in the ring the oscillation of which 

 is the subject of investigation. The rest of the negative 

 electrons may either describe their isolated orbits, or may 

 form some other regular systems. These will evidently 

 cause disturbances in the ring, which I have not calculated, 

 inasmuch as it was necessary to introduce further assump- 

 tions as to the orbits or the arrangement of electrons. As 

 a dynamical system illustrating the regularity in spectrum 

 lines, I have assumed the ring and the central positive 

 charge as an ideal atom, leaving the remaining electrons 

 outside the domain of investigation. 



Further evidence as to the validity of such an hypothesis 

 is afforded by the fact that spectrum lines of most elements 

 are not all subject to a regular law, but that there are a 

 great many characteristic lines the position of which is not 

 expressible by a simple mathematical formula. Moreover, 

 the complexity in the structure of spectrum lines will prob- 

 ably find simple explanation by the perturbations due to 

 stray electrons. These considerations point to the view 

 that only a small part of the negative electrons accompany- 

 ing an atom should be arranged in a ring, in order to 

 approximate to the real state of a chemical atom. 



There is no doubt that Prof. Schott will have good oppor- 

 tunity of applying the mathematical investigations of 

 C. Neumann and Hicks in discussing the oscillations of 

 three rings, just as Lindemann made use of Lamp's func- 

 tions in studying the spectrum lines as due to the vibration 

 of ellipsoidal atoms, but I am afraid that the result will be 

 difficult of interpretation. 



I take this opportunity of introducing a small omission 

 in my former letter to N.ature for February 25 (p. 392, 

 second column, line 31 from bottom); after "in opposite 

 senses " add " in a magnetic field." 



