February 13, 1908J 



NA TURE 



343 



case no new type can possibly arise, and every plant and 

 animal in the world is an " end group," which is utterly 

 inconceivable from the evolutionary hypothesis. 



Sunlight is pure and colourless. Under the stimulus 

 of a prism it becomes red, yellow, and blue. If animal 

 form and colour are no more than the prismatic separa- 

 tion of inherent characters preexisting in the germ plasm, 

 it seems to me that the theory of " Darwinian " evolution 

 falls to the ground, and that it is not logical to use 

 arguments founded on that hypothesis to establish con- 

 clusions that a-e fatal to its existence ; but I write with 

 a certain trepidation, remembering the fate of the earthen- 

 ware pipkin that ventured into the stream amongst the 

 iron pots. E. C. Spicer. 



\\'ater>tocI<, Oxford, February i. 



The slightly dogmatic tone of my original article 

 (January 2, p. 193) under the above heading has called 

 forth quite a number of confessions of failure to under- 

 stand the modern attitude towards this question. But, 

 though we admire the generous spirit of those who have 

 come forward and made a public exhibition of this failure, 

 we consider that we have contributed our fair share by 

 enticing them out into the open, and that they are asking 

 too much when they try to relieve their very natural 

 embarrassment by appealing to us to tell them what the 

 modern attitude really is. 



Ideally, of course, those who by inclination or accident 

 are in touch with recent thought on these subjects ought 

 to be only too glad to impart what they know to others 

 less fortunate — to the aged and to the remote. But prac- 

 tically it cannot be done. The Editor of Nature would 

 say, perfectly rightly, that the correspondence column of 

 his journal was not the place for enlightening those who 

 fail to keep abreast of modern biological thought. 



Dr. .Archdall Reid's statement of the real nature of the 

 problem is not a final one of course (as he probably thinks 

 it is), but it is undoubtedly an improvement on the chest- 

 nut-old one which asserts that acquired characters are 

 inherited as well as innate ones — a statement which is 

 meaningless, because all characters are both acquired and 

 innate. 



If Dr. Bastian and Mr. Spicer have read that part of 

 Dr. Reid's book, " The Principles of Heredity," which 

 deals with this subject, their letters show that they have 

 been unable to understand it. If they have not, it docs 

 not seem to us to be profitable to discuss the matter until 

 they have. A. D. D. 



Atmospheric Electricity and Fog. 



In view of the interest recently shown in the subject 

 of the dispersion or prevention of fog, it may be opportune 

 to direct attention to a recent remarkable example of an 

 atmospheric electricity phenomenon which usually accom- 

 panies London fogs. I should first explain that the method 

 adopted at Kew for determining the absolute value of the 

 potential gradient — i.e. the increase in the voltage per 

 metre of height above the ground — certainly does not err 

 in the direction of overestimating it. Taking eight years, 

 1898 to 1904, I found in a recent paper ' that the mean 

 value of the potential gradient at Kew was 159, the mean 

 value for January being 201. The phenomenon referred 

 to above is the occurrence during fog of specially high 

 positive potentials, values double or treble that appropriate 

 to the season being not unusual. At such times, how- 

 ever, there are usually large and frequent oscillations in 

 the value of the gradient, so that the maintenance of an 

 e.xceptionally high value for a number of consecutive hours 

 is comparatively rare. On the morning, however, of 

 January 21, during an intensely thick fog, the potential 

 gradient at Kew exceeded 730 continuously from i to 

 9.30 a.m. How much it may have exceeded this value 

 it is impossible to say, as the trace was beyond the limits 

 of registration during the whole of this time. Both before 

 the trace left the sheet and after its return the oscill.a- 

 tions in the potential gradient were large, so that the 

 maximum value was probably at least 1000. 



A question of practical interest is whether the steepness 

 of the potential gradient near the ground during fogs 

 1 Phil. Trans.. A, vol. ccvi., p. 299. 

 NO. 1998, VOL. 77] 



serves, or may be made to serve, a useful purpose in help- 

 ing to clear the atmosphere of dust and smoke. It would 

 also be interesting to know whether these high potentials 

 are wholly without physiological effects on the human 

 body. CiMRLES Chree. 



National Physical Laboratory, Februarv 5. 



The Penetrating Radiation. 



Many writers apparently assume that the- penetrating 

 radiation is due to 7-like rays coming from radio-active 

 products in the ground, and is practically constant in 

 amount. It seems probable, however, that the penetrating 

 radiation comes largely from radio-active products in the 

 air, and that it fluctuates greatly in value. 



Taking the mean value found by Strutt and Eve for the 

 radium content of sedimentary rocks as 0.9(10)-'- grams 

 of radium per gram of rock, one finds that it is the source 

 of y radiation which would produce an ionisation on the 

 surface of the ground in air of less than 0-8 ion per c.c. 

 per sec. Now the above value for the radium content is 

 perhaps large for surface soils subject to constant erosion. 

 The actual value found by Cooke for the ionisation in air 

 as due to the penetrating radiation was 4.5 ions per c.c. 

 per sec. iWcClennan takes the value as 9, and the writer 

 has found a much larger value in the open country during 

 the warm hours of the day. Assuming that the emanation 

 of the radium differs from a depth of 50 cm. or 60 cm. of 

 the ground, one gets a penetrating radiation that will 

 produce a much greater ionisation. 



If the penetrating radiation is due to radio-active pro- 

 ducts in the air, one would expect that it would vary 

 very greatly in amount. The experiments of Jaffe, Camp- 

 bell, Wood, Borgmann, the writer, and others would 

 indicate this. On the other hand, if the penetrating 

 radiation comes from radio-active products in the ground, 

 its amount should be quite constant. Dike has found that 

 the active deposit which gathers on a charged wire ex- 

 posed to the air varies greatly with the time of day. Eve, 

 by his charcoal method, has found widely different amounts 

 of the radium emanation in the air at different timer.. 

 The writer (Science, July 12, 1907) has found that during 

 a heavy rain or snow the penetrating radiation decreases 

 very greatly in amount. Rain and snow have been shown 

 to carry down radio-active products, and if the penetrating 

 radiation is due to radio-active products in the air, then its 

 value should be less during a heavy rain or snow. 



If the penetrating radiation is due largely to radio-active 

 products in the air, its value in underground cavities 

 should be less than on the surface of the ground. This 

 is what Elster and Geitel found. The writer has found 

 the ionisation in a closed electroscope to be approximately 

 the same (a) in a cave ; (b) in a cistern where there was 

 4 feet of water on all sides of the electroscope ; and (r) 

 inside a screen of lead and cast-iron blocks. In the open 

 country during August and September (1907) this same 

 electroscope showed an ionisation during the day some 

 three or four times greater than during the night. In the 

 cave and cistern the ionisation during the day and night was 

 the same. It is natural to suppose that the penetrating 

 radiation was greater during the day, and was due to 

 radio-active products which had diffused out from the 

 ground. During the night the ionisation was not much 

 greater than for the electroscope in the cave or cistern. 



Johns Hopkins University. W. W. Strong. 



Classification of Secondary X-Radiators. 



The relation between the character of secondary X-radia- 

 tio'n emitted by elements when subject to the same beam 

 of X-rays and the atomic weight of the radiating sub- 

 stance has been considered in various papers, but only 

 brief reference has been made to the dependence of the 

 character of the secondary on that of the primary radia- 

 tion. We have recently made a more systematic study 

 of the relation between the secondary and primary rays. 



Although the behaviour of no two substances is exactly 

 the same under the same conditions, yet substances may 

 conveniently be divided into several groups, each consisting 

 of elements which emit a radiation possessing many 

 properties characteristic of that group. 



