January 2, 1908] 



NATURE 



197 



work of Ross, Grassi, and others during the last few 

 years, we find him talking of Eucalyptus globulus, 

 and telling us that " Lavcran and others claim that 

 the disease is produced by certain microorganisms 

 allied to the Flagellata, but this is contested by 

 Tommasi-Crudeli. " Certainly this is a book that 

 requires to be read with discrimination. C. S. 



Etude sur les Foudroiements d'arhres constatis en 



Belgique pendant les Annees 1884-1906. By E. 



\'anderlinden. Pp. 79. (Bruxclles : I^'Observatoire 



Royal de Belgique, 1907.) 

 A PHENOMENO.s' of Very frequent occurrence, and one 

 that has been much studied, is the injury occasioned 

 to trees by lightning, and yet we have no satisfactory 

 explanation of the many difficulties that the problem 

 presents. It must be admitted that these inquiries 

 have not always been pursued on scientific lines, and 

 the facts that have been collected and the theories that 

 have been suggested need a thorough sifting and ex- 

 amination. This is the task to which M. Vander- 

 linden has applied himself with very considerable suc- 

 cess. If he has not satisfied curiosity in all direc- 

 tions, he has at least overthrown some popular errors, 

 limiting and indicating the field of strictly useful 

 inquiry. 



It has long been held that some trees are not liable 

 to be struck by lightning. M. Vanderlinden examines 

 this assertion, and finds that in Europe no kind of tree 

 is free from this damage. Some kinds, sucli as the 

 oak, the poplar, and some species of fir, are more fre- 

 quently injured than others, but there are not sufficient 

 data to show how far this result is effected by situ- 

 ation, by size, by exposure, or by the greater number 

 cultivated. The causes which may be expected to in- 

 fluence the selective power of lightning are also investi- 

 gated, but with only negative results. Among these 

 we find the character of the soil, the cliemical consti- 

 tution of the timber, the proximity of water surface, 

 the formation of the leaf, the shape of the tree, and 

 particularly the character of the bark. In this last, 

 the author thinks that possibly we have a source of 

 explanation that has not been sufficiently examined. 

 Trees possessing a smooth baric, such as beeches, are 

 less liable to be struck than those having irregular, 

 rugged, roughened surfaces, the external portions of 

 which, becoming dry and dead, are bad conductors of 

 olectricitv. 



Other questions discussed are the manner in which 

 lightning affects the tree, the character of the so- 

 called spiral injuries, the actual combustion of the 

 tree material, and the manner in which men and 

 animals suffer injury when in the neighbourhood of 

 trees that have been struck. The whole forms an 

 admirable examination of a ver)- scattered literature, 

 and accurately exhibits the present position of an un- 

 exhausted inquiry. Detailed tables accompany the 

 paper giving information concerning the locality, the 

 number, and variety of trees, reported as being struck 

 by lightning in the vears under review. 

 The Laws of Healtli. .1 Handbook on School Hygiene. 



By Dr. Carstairs G. Douglas. Pp. vii + 240. 



(London: Blackie and .Son, Ltd., 1907.) Price 3s. 



net. 

 This should prove a useful book for school teachers 

 and others interested in the important subject of school 

 hygiene. Anatomical and physiological details are 

 briefly dealt with, as the author justlv remarks that 

 there arc several excellent manuals dealing with these 

 subiects. On the other h.ind. the reallv important and 

 fundamental portions of hygiene as applied to school 

 life, such as ventilntion, warming and lighting, and 

 the fittings of the school buildings, the nutrition, ail- 

 ments and deformities of the pupils, are discussed with 

 sufficient fulness, and these sections are copiously 

 NO, 1992. VOL. 77] 



illustrated with diagrams and drawings. In dealing 

 with fatigue, we are glad to see the stress the author 

 lays on a proper amount of sleep, and that he ascribes 

 a good deal of the listlessness and inattention, &c., met 

 with in public elementary schools to deficient sleep. 

 We have nothing but praise for the chapters on the 

 slight ailments of children, the eye and ear, and infec- 

 tious diseases; even the " fourth disease " is referred 

 to. The only omission we have noted is a reference 

 to the cleansing and periodical disinfection of school 

 premises. R. T. H. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himseij responsible for opinions 

 expressed by his correspondents. Neither can he tindertake 

 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.] 



The Wehnelt Kathode in a High Vacuum. 



The interesting experiments on the Wehnelt k.nthode 

 in a high vacuum described by Mr. Soddy in Nature for 

 November 21, 1907 (p. 53), do not, I tliink, necessarily 

 conflict with the experiments of either Dr. Wehnelt or 

 myself, nor do they appear to vitiate the conclusions we 

 drew from them. 



The experiments by which Dr. \\'chnclt showed that 

 the negative leak from a lime-covured kathode was in- 

 dependent of the gas pressure were made with currents 

 much smaller than those used by Mr. Soddy. When 

 currents of the order of an ampere per square centimetre 

 of kathode surface are obtained, the phenomenon appears 

 to be accompanied by a glow round the kathode implying 

 that ionisation in the gas itself is playing a part. Prof. 

 J. J. Thomson (" Conduction of Electricity through 

 Gases," second edition, p. 477) has shown' that the 

 potential fall required to produce ionisation by collisions 

 with these large currents decreases rapidly as the current 

 increases, so that even the comparatively small fall of 

 30 volts quoted from Wehnelt 's paper would be sufficient 

 to produce a considerable effect of this kind. 



When the pressure of the gas falls below the value re- 

 quired to maintain this glow under the assigned con- 

 ditions as to difference of potential, pressure, &c., there 

 will be a sudden drop in the value of the discharge 

 current. This appears to be what Mr. Soddy has 

 observed. 



I do not wish to imply that the negative emission is in 

 all circumstances independent of the presence of gas in 

 the tube. It is well known that the negative discharge 

 from hot metals is particularly sensitive to hydrogen. In 

 a recent paper (Phil. Trans., A, vol. ccvii., pp. 53 

 el seq.) I have described experiments which seem to prove 

 conclusively that the leak from hot platinum is not due 

 to traces of absorbed hydrogen. .\s the experiments in 

 gases other than hydrogen show that the negative leak 

 from hot platinum is independent of the pressure at low 

 pressures, it is very unlikely that it is caused primarily 

 by traces of anv other of the common gases. These 

 gases only appear to increase the fundamental electronic 

 discharge from the hot kathode by a process involving the 

 occurrence of ionisation by collisions. 



The foregoing explanation of Mr. Soddy 's result is only 

 offered as a suggestion, for two reasons. In the first 

 place, Mr. Soddy's letter is not sufficiently definite as to 

 some of the salient facts, as, for example, the magnitude 

 of the pressure before the current dropped. In the second 

 place, there is a good deal about the behaviour of these 

 Wehnelt kathodes which is imperfectly understood, and 

 which merits further investigation. 



O. W. Richardson. 



Princeton, N.J., December 7, 1907. 



I AM glad to learn Prof. Richardson's view is much 

 the same as that expressed in my letter to Nature of 

 November 21, 1907, that the currents in question are 

 carried mainly by the residual gas, not by the electrons 

 expelled from the kathode. Into the views that have been 

 expressed in earlier communications it is therefore un- 



