340 



NA TURE 



[January 21, 1909 



the galls, and refers (by name simply) to the gall- 

 forming animals; he arranges the galls according to 

 the families of plants affected ; he supplies more than a 

 thousand serviceable illustrations, a statement of the 

 geographical distribution of each gall, and the indis- 

 pensable bibliographical references. The second volume 

 is in the press ; the first volume deals with the galls of 

 cryptogams, gymnosperms, monocotyledons, and the 

 dicotyledons from. Ranunculacete to Rosaceae. The 

 work will be a great boon to entomologists, botanists, 

 foresters, and agriculturists. We hope that the author 

 will not write ixnis to his magnum opus without dis- 

 cussing, as he is so competent to do, the fascinating 

 biological problems which are raised bv the study of 

 galls, crowning his work of description with an essav 

 of interpretation. 



Practical Coastal Navigation, including Simple 

 Methods of finding Latitude, Longitude, and 

 Deviation of Compass. Bv Comte de Miremont. 

 Pp. 88. (London : J. D. Potter, 1908.) Price 4s. 

 In this small volume Comte de Miremont has collected 

 an enormous amount of useful ■"^■"-mation and what 

 might be called tricks of the trade, which tell the 

 young navigator everything that has been found use- 

 ful in coastal navigation after years of experience. 



The book is excellently arranged, and the explana- 

 tions are simple. Besides the various chapters on 

 actual coastal navigation, deviation and rule of the 

 road, and weather forecastmg in home waters, are 

 most ably explained. 



The book should find a place in everv chart-house 

 and navigation school. Comte de Miremont is to be 

 highly congratulated on having produced such a useful 

 aid to mariners, and to those wishing to become 

 efficient in this particular art. H. C. Lockver. 



LETTERS TO THE EDITOR. 



[The Editor does 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 anonymotts communications.] 



The Radiation of the Active Deposit from Radium 

 through a Vacuum. 



When the radium emanation is transformed into radium 

 A, the process is accompanied by the emission of a particles 

 with a velocity of i-7ox 10° centimetres per second (Ruther- 

 ford, Phil. Mag., October, 1906). The portion of the atom 

 from which the a particle has been emitted, which con- 

 stitutes the radium .\, must therefore be subjected to 

 considerable shock and recoil in a direction opposite to 

 that in which the a particle is projected. If we further 

 consider that the mass of the a particle is 4(H = i), and 

 that of the active deposit of the order 100, it follows that 

 at the moment of its formation this product must be 

 travelling with a velocity of the order 10' centimetres per 

 second. In ordinary circumstances, when the emanation 

 is mixed with air at atmospheric pressure, the radium A 

 particle will possess only sufficient energy to permit it to 

 travel a fraction of a millimetre before being stopped by 

 collision with air molecules. On the other hand, at very 

 low pressures, these particles should travel considerable 

 distances without being stopped by the rarefied air, and 

 come to rest on the enclosure containing the emanation. 

 These particles should, in fact, constitute a type of very 

 easily absorbed radiation. It has been the object of some 

 experiments which we have recently performed to demon- 

 strate directly the existence of this radiation. 



The emanation from a fairly large quantity of radium 

 was condensed at the bottom of a wide glass tube by 

 immersing its end in liquid air. .\ brass plate, which 

 just fitted into the glass tube, was suspended, in a high 

 vacuum, a few centimetres above the condensed emana- 

 tion so ris to expose it to the bombardment of the active i 



NO. 2047, VOL. 79] 



deposit particles being fired up the tube. After a suitable 

 exposure the plate was removed, and its activity tested in 

 the usual manner by a quadrant electrometer. The surface 

 of the plate exposed to the emanation was always found 

 to be highly radio-active. 



Now this in itself would afford no evidence of the effect 

 sought, for it is well known that when a large quantity 

 of radium emanation is condensed in liquid air, the con- 

 densation is by no means complete, and there always exists 

 in the vessel, above the condensed emanation, a consider- 

 able quantity of emanation in the gaseous state. .\ plate 

 situated above the emanation as described above must 

 therefore of necessity become radio-active on this account. 

 But it was always found that the activity of the surface 

 of the plate facing the emanation was greater than that 

 of the opposite side, and it seems quite certain that this 

 excess of activity is due to the direct radiation of the 

 active deposit on to the plate. The ratio of the activity 

 of the surface turned towards the emanation to that turned 

 away from it has been found, under suitable conditions, 

 to be as great as 50 to 1. The exact ratio obtained 

 depends, of course, on a variety of experimental conditions, 

 but in all circumstances the activity of the surface of the 

 plate turned towards the emanation exceeded that of the 

 reverse side. Moreover, by interposing a screen between 

 the emanation and the plate, the excess activity collected 

 on the surface of the plate turned towards the emanation 

 could be completely obliterated. Experiments have been 

 made at different pressures, and it has been found that 

 the radiation is cut down to one-twelfth by traversing about 

 8 centimetres of air at a pressure of 1-15 inillimetres of 

 mercury. The same distance of air at 2 millimetres 

 pressure is sufficient almost completely to stop the radiation. 



These experiments give rise to a number of interesting 

 questions which it is not yet possible to answer with any 

 certainty. In the first place, it seems probable that when 

 I he emanation is condensed at the bottoin of an evacuated 

 lube, the attendant phenomena must be somewhat com- 

 plicated, for when in radio-active equilibrium the emana- 

 tion will be mixed with all its decomposition products. 

 .'\t every stage in the radio-active series at which o par- 

 ticles are expelled, some of the residual atoms should be 

 fired up the tube. .Although it is not yet possible to speak 

 with certainty, it would seem that both radium .\ and 

 ladium B arc projected up the tube on to the plate ex- 

 posed to the r.idiation. 



.Another question of importance also arises as to whether 

 the particles projected from the emanation are charged or 

 not. Some experiments have already been made on this 

 point with the object of deflecting the radiation by an 

 electric field ; but the difliculties are considerable, and no 

 definite evidence has vet been obtained. We hope, how- 

 ever, that these difficulties will not prove insuperable. 



S. Russ 

 W. Makower. 



Physical Laboratory, The University, Manchester, 

 January o- 



The Isothermal Layer of the Atmosphere. 



It seems to me that in Nature of January 7 (p. 281) 

 Mr. Dines iuccessfullv defends his simple, compact, but 

 extremely efticient apparatus from the suspicions that have 

 been levelled at it. The tests of the instrument before 

 and after use show that it truly records the temperatures 

 and pressures to which it is reduced. Mr. Dines is there- 

 fore entitled to call for adequate discussion of the most 

 marked outcome of the experiments — the fact that in nearly 

 all cases the minimum reading of temperature is reached 

 long before the maximum height in the ascent, and long 

 after in the descent. To suggest that the thermometer or 

 the barometer may be slightly out is really to evade the 

 problem. 



Taking, then, the readings as fairly accurate, do thov 

 prove the "isothermal layer"? What are the circum- 

 stances ? To the best of mv knowledge they are these ; — 

 the instrument is screened by a polished metallic cylinder 

 open at top and bottom, the centre of which it occupies, 

 and the draught of air produced by the up-rush and down- 

 rush of the balloon is relied on to ensure that the thermo- 

 graph, which is of light metal strip, shall take the 



