January 26, 191 1] 



NATURE 



4ai 



tion. In this small book he now gives a connected 

 statement of all his work. 



The phenomena attending the passage of electricity 

 through gases are in many cases ver}- complex, but 

 it has certainly been evident of late years that the 

 fog which has covered the field of exploration is be- 

 ginning to lift. Here and there we are able to see 

 clearly for a little way and to grasp the relations of 

 various points to one another. The simple and satis- 

 factor}- theorv^ of ionisation by collision, which Prof. 

 Townsend has worked out, is an instance of this 

 improvement. He shows in the first chapter of hi§ 

 book how electrons set free by the action of ultra- 

 violet light or other agents from one wall of an 

 ionisation chamber grow in number as they are 

 enided across the chamber by a sufficient electric 



■rce. Collisions with gas molecules add fresh elec- 

 ons to the stream, and when the force is not too 



reat the number which eventually reach the opposite 



all is an exponential function of the width of the 

 hamber. He bases his explanation on the assump- 

 tions that (i) an electron must acquire a certain velo- 

 city before it can ionise a gas molecule by colliding 

 with it ; (2) a successful collision adds one, and only 

 one, electron to the stream; (3) an electron after a 

 collision, successful or not. has lost all the energy- it 

 previously possessed, and starts its career afresh 

 These assumptions can hardly be quite accurate, and 

 the remarkable agreement between the calculated and 

 the experimental results seems almost more than there 

 is any right to expect. It is quite a satisfaction to 

 find that the agreement does not hold in extreme 

 cases, and that the failure is, as the author points 

 out, in the right sense. The third assumption is cer- 

 tainly not always true; Prof. Townsend has himself 

 shown, in later papers not discussed in this book, that 

 an electron can acquire considerable energy in an 

 electric field when moving through a very dry gas; in 

 other words, that the electron does not then give up 

 all its energy at each collision. Again, it is interest- 

 ing to find that electrons are not to be supposed to be 

 incorporated with the atoms with which they collide; 

 or at least that it has been found possible to ignore 

 such an effect. If the idea is a correct one, it seems 

 unlikely that j8-rays can ever be incorporated w^th 

 atoms with which they collide. Thus the undoubted 

 success of Prof. Townsend's theor}- opens up further 

 questions of great interest. 



In the second chapter it is shown that the positive 

 ions must acquire far more energy- than the negative 

 before they can ionise. It is only when the electric 

 force is very- great that the influence of the positive 

 ion is perceptible. When, however, the force reaches 

 a certain value the combined action of the positives 

 and the negatives is sufficient to multiply a small 

 initial ionisation indefinitely, and there is a "dis- 

 charge." The " sparking potential " can be calcu- 

 lated from the ionising coefficients of positives and 

 negatives, as previously found by experiment, and 

 here again there is an excellent agreement between 

 calculation and actual test. A careful explanation is 

 also given of the difference between the sparking 

 potential and the potential necessan,- to maintain a 

 discharge once started. 



NO. 21/^2. VOL. 8a1 



The argument of the book iy generally quite clear, 

 but there are occasional obscurities. On p. 23, for 

 example, the statement is confused, though essentially 

 accurate of course. ** The element dy of these paths * 

 is not a proper phrase. 



The book is a welcome record of very useful and 

 interesting work. 



TWO PHOTOGRAPHIC ANNUALS. 

 (i) Penrose's Pictorial Annual. The Process Year 



Book. Edited by W. Gamble. Vol. xvi., 1910-11. 



Pp. X+192. (London: A. W. Penrose and Co., 



Ltd., n.d.) Price 5s. net. 

 (2) The British Journal Photographic Almanac, 191 1. 



Jubilee issue. Edited by George E. Brown. Pp. 



1348. (London : Henry Greenwood and Co., n.d.) 



Price IS. net; cloth, 15. 6d. net. 

 (ir^T^HE Process Year Book" has for its 

 * -■- object the display of specimens of work 

 done by each of the many and various processes of 

 reproduction. Care is taken that each process is 

 represented by a sample obtained with the maximum 

 of efficiency of that process. The volume thus gives 

 the reader an idea of the standard of the workman- 

 ship of to-day attained in each case, and also a com- 

 parison between the different kinds of results that 

 can be secured. 



There is no doubt that many of the processes of 

 reproduction of to-day are really very fine, and a 

 glance through these pages will probably make the 

 reader think that it seems scarcely possible to pro- 

 duce better work. Yet those who are closely asso- 

 ciated with the subject, and they are the people who 

 know the true failings, take a somewhat pessimistic 

 view. Thus the editor in last year's annual was of 

 the opinion that the beautiful processes were on the 

 downward grade, and in this volume he states "it 

 cannot be said that the situation is much changed." 

 The race for speed and large output, coupled with 

 no time or desire to experiment, are among the 

 reasons he gives for this halt, cw rather retrograde 

 movement. 



Nevertheless the volume before us demonstrates 

 that a ver\' high stage of efficiency has already been 

 reached, and it is possible . that because such rapid 

 progress in advancement as previously made is not 

 maintained now, this pessimistic view is held. 



The amount and quality' of the work embodied in 

 this volume is a credit, not only to the editor, Mr. 

 William Gamble, but to the publishers, Messrs. Percy 

 Lund Humphries and Co., Ltd., and the proprietors, 

 Messrs. A. W. Penrose and Co., Ltd. A large 

 number of brief but interesting chatty articles on 

 various branches of the subject are interspaced amcwig 

 the large number of illustrations, and the variety 

 and high standard of the latter are to be highly 

 commended. 



Every trouble has been taken to give credit to those 

 who have contributed to the volume, and it may be 

 said that this issue even excels the very excellent 

 volumes which have been noticed before in these 

 columns. 



Not only will the book be of high interest to all- 



