544 



NATURE 



[February 22, 191 



however cons^enial to him, must have caused him 

 a pood deal of labour and anxiety. And the work 

 has been done in the best possible way. The short 

 notes appended here and there are very helpful, but 

 the personality of the editor is never obtruded ; in- 

 deed, it might sometimes, with advantage perhaps, 

 have been more in evidence. The error, if it be one, 

 is on the right side, and it may be that in connection 

 with the selection of letters which we understand 

 it is proposed to publish, we may have more of the 

 editor's views on the various scientific questions, still 

 unsettled, which are raised in these volumes. 



The present volume contains papers on the elec- 

 trification of air, on contact electricity, on radio- 

 activity, on navigational and tidal instruments, terres- 

 trial magnetism and the correction of the compass, 

 and one or two of Lord Kelvin's last popular ad- 

 dresses. These papers are for the most part too recent 

 for discussion here; the continued progress of re- 

 search on ionisation and radio-activity is a commentary 

 on many of them, and in any case we have no space 

 for even a descriptive account of their content. 



A word may perhaps be said on the vexed question 

 of contact electromotive force between metals. Surely 

 the reality of any contact difference of potentials must 

 depend on whether or not any perceptible energy 

 change — evolution or absorption — takes place at the 

 surface, or rather the locality, of contact. When a 

 part of a circuit carrying a current is moved in a 

 magnetic field work is done upon or by that part, and 

 energy is there absorbed or evolved, while in all parts 

 of the circuit there is energy converted into heat with 

 a corresponding- potential gradient. And when there 

 is change of potential from metal to liquid or from 

 liquid to metal, or from liquid to liquid, energy 

 changes which correspond are concerned. The pro- 

 cess is, if anything, still more evident in thermo- 

 electric circuits, and leads to an absolute evaluation 

 of electromotive force. Now there must at bottom 

 be unity of cause of electromotive force, and the exist- 

 ence of a finite contact electromotive force between 

 metal and metal, which seems to have been insisted 

 on to the last by Lord Kelvin, is, from the point of 

 view of the energetics of the circuit, a great difficulty 

 to some at least of those who are earnestly striving 

 to obtain a clear and consistent view of voltaic 

 phenomena. 



The papers on radio-activity are very interesting as 

 heroic attempts to explain on dynamical grounds, 

 without calling into play any process of atomic dis- 

 integration, the extraordinary energy changes which 

 radium and its products have disclosed. The com- 

 parison (p. 209) of the permanent rise of temperature 

 of a thermometer which has radium near its bulb 

 with the excess of temperature of a vessel containing 

 bfack cloth over that of a similar vessel containing 

 white cloth, when both vessels are immersed in baths 

 of water exposed to the sun's rays, seems only to 

 make the difficulty of any such explanation more 

 evident, at least to the mind of the ordinary student. 

 What corresponds to the sun's radiation in the radium 

 experiment? According to this hypothesis, the radium 

 " somehow " picks up energy of ethereal waves in a 

 very special and effective manner. Is it not 

 NO. 2208, VOL. 88] 



simpler and more reasonable to put down in 

 ordinary way the energy evolved to the chs 

 effected in the stuff, especially as such changes 

 chemically manifest? It would seem so; and yetj 

 suggestion must be received with resf>ect and 

 as far as possible, even though it appear to ini 

 the reconsideration of many other cases of phyi 

 and chemical change. 



The navigational and tidal papers ar- I 

 tional, but are of great interest in connection 

 terrestrial physics. The tide-predicting machine, 

 from the results of the analysis of tide curves, 

 the past disclose the future in a very remj 

 manner, and if nothing else had directly or inc 

 come out of the work of tl^e British Association 

 mittee on Tides, would have justified the whole] 

 penditure on that subject. But much more has 

 through Lord Kelvin's and Sir George Darwin's 

 ance and the work of the committee and its si 

 calculators the book of the tides of long period 

 been opened, and its secrets are being read. 



As a lesson on the theory and practice of m« 

 ical integration the tidal analyser ought to be 

 known than it is. We have not seen, except in 

 nection with the tidal application, any adequate popi 

 lar account of it ; and yet the machine, from the poir 

 of view of integration-theory, and even of differentu 

 equations, is of great interest and importance. Th 

 machine with two complete harmonic components i 

 periods of the ratio 1:2 is, or was, used for variation 

 of atmospheric pressure ; but the large machine cor 

 structed for the tidal application and described in th 

 volume of papers before us stands inactive at Sout 

 Kensington. 



The oration on James Watt and the inaugTJK 

 address delivered as chancellor of the University c 

 Glasgow are given as interesting memorials of Lor 

 Kelvin's lifelong association with Glasgow. The 

 were reported in Nature at the time, but their reprc 

 duction here gives a fitting personal note with whic 

 to close the long and brilliant record of scit ' 

 work which these volumes contain, a record v 

 is one of the most glorious scientific facts of the • 

 scientific age of the world's history. A. 1 



SEWAGE PURIFICATION. 



Modem Methods of Sewage Purification : a Guidi 

 the Designing and Maintenance of Sewage Purr 

 tion Works. By G. Bertram Kershaw. Pp. n 

 356. (London : Charles Griffin and Company, ! 

 191 1.) Price 215. net. 



IN view of the numerous text-books dealing 

 the purification of sewage which have rec 

 appeared it would seem difficult to justify the pul 

 tion of a further work on the subject. Durinj 

 long period in which the author has been assc 

 with the Royal Commission on Sewage Disposal' 

 has had exceptional opportunities of personally iiv 

 tigating the various methods which have been adof 

 for the solution of the difficult problem of the econt 

 disposal of sewage, and consequently he is well quali| 

 fied to undertake a treatise on the subject. 



The book to a very large extent, however, c 

 its justification to the systematic and exhaus 



