July \6, 1908J 



NA TURE 



245 



that of the Royal Botanic Gardens, Kew, lie within 

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 flbout each of the very large number of London 

 libraries is just what a student wants to assist him 

 in his search for boolvs on a particular subject. Few 

 jjersons. unless they have made special inquiries, can 

 have any idea of tlie immense number of books avail- 

 able in London for reference by the seeker after 

 knowledge or recreation. Students owe a debt of 

 gratitude to the Senate of the Universitv of London 

 for giving instructions for the preparation of this 

 g-uide, and to Mr. Rye for his complete understanding 

 of their needs. 



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 0/ Nature. 

 No notice is taken of anonymous communications.] • 



The Spectrum of Radium Emanation. 



In Nature of July g a letter from Prof. Rutherford 

 appears giving an excellent corroboration of measure- 

 ments of the spectrum of radium emanation which we 

 communicated to the Royal Society on July 1. 

 There can, therefore, be no doubt of the accuracy 

 of Prof. Rutherford's measurements. When Mr. 

 Watson, who is engaged in measuring accurately with a 

 lo-foot grating the secondary spectrum of hydrogen, has 

 finished his task, we shall be able to introduce some 

 small corrections in our figures. William Ramsay. 



A. T. Cameron. 



University College, Gowcr .Street, London, W.C., July 9. 



The full account of these researches will shortly appear 

 in the Philosophical Magazine. O. W. Richardson. 



Princeton, N.J., July 3. 



The Kinetic Energy of the Ions emitted by Hot 

 Bodies. 



In a paper communicated to the .American Physical 

 Society at the New York meeting on February 29, the writer, 

 in collaboration with Dr. F. C. Brown, showed that the 

 part of the translational kinetic energy of the negative 

 ions emitted by liot platinum, whicli depends on their 

 component of velocity normal to the emitting surface, has 

 the same mean value as the corresponding quantity for a 

 molecule of gas at the teinperature of the metal, and, 

 further, that that component of the velocity is distributed 

 among the different ions according to Maxwell's law of 

 distribution of velocity among the molecules of a gas. 

 Since then Dr. Brown has succeeded in showing that the 

 same laws hold for the positive ions emitted by hot 

 platinum. 



Using a different method, the writer has succeeded in 

 measuring the portion of the kinetic energy of the ions 

 which depends on their component of velocity parallel to 

 the emitting surface. Within the limits of experimental 

 error, this quantity has the same mean value, for both 

 positive and negative ions, as the corresponding quantity 

 for a molecule of gas at the temperature of the metal, and 

 is distributed among the different ions according to 

 Maxwell's law. 



Taken together, these investigations show that the ions 

 emitted by hot platinum, under normal conditions, are 

 identical, as regards their kinetic properties, with the 

 iTiolecules of a gas of the same molecular weight, at the 

 temperature of the metal. It follows, bv an application 

 of the kinetic theory of gases, that the same thing holds 

 for the free electrons inside the metal. This result has an 

 important bearing on the electron theory of metallic con- 

 duction and of the emission of electromagnetic radiation 

 bv hot bodies. 



This Is the first direct experimental confirmation of 

 Maxwell's laws relating to the distribution of velocity 

 among a collection of moving particles in a state of 

 statistical equilibrium. 



NO. 2020, VOL. 78] 



Absorption of X-Rays. 



So.ME of the most interesting observations made in the 

 investigation of the properties of homogeneous beams of 

 Rontgen radiation are those exhibiting the connection 

 between the absorption of X-rays and the emission of 

 secondary X-rays from the absorbing substance. Many 

 elements — probably all — when subject to a suitable primary 

 beam, are the source of a homogeneous Rontgen radiation 

 which is characteristic of the element emitting it. The 

 following conclusions concerning the emission of this radia- 

 tion have been found to be perfectly general, so far as 

 experiments have been made. 



When a very absorbable primary radiation is incident on 

 a given element, the homogeneous radiation characteristic 

 of that element is not emitted in appreciable Intensity. 



,^s the general penetrating power of the primary radia- 

 tion is gradually increased, the absorption decreases only 

 up to a certain point. When the penetrating power 

 becomes greater than that of the radiation characteristic 

 of the absorbing element, the absorption of that primary 

 radiation begins to increase, and a secondary homogeneous 

 radiation begins to be emitted. Then there is a rapid and 

 considerable Increase in both the absorption of the primary 

 rays and In the emission of secondary rays. When the 

 general penetrating power is increased still further, the 

 absorption decreases again in the usual way, and the 

 intensity of secondary radiation decreases at the same rate 

 — in some cases at least — as the ionisation produced by 

 the primary beam in air. 



The special absorption of the primary rays thus con- 

 nected with the emission of secondary rays is a con- 

 siderable fraction of the total absorption — thus in iron the 

 increase is about double the absorption previous to the 

 emission of the rays. 



Experiments have not been made to determine if all the 

 extra energy absorbed appears as energy of secondary 

 radiation, but from observations of the absorbability of 

 the secondary radiation and of the ionisation it produces, 

 It appears probable that a large proportion is re-emitted. 



The energy re-emitted In the form of a radiation of more 

 absorbable type is in some cases sufficient to make the 

 total ionisation produced in an electroscope placed imme- 

 diately behind a thin absorbing sheet of metal greater than 

 that produced by the direct unabsorbed primary beam. 



The emergent radiation is then a mixture of two homo- 

 geneous radiations, the proportions of which depend prin- 

 cipally on the coefficients of absorption of the incident 

 radiation, and of the radiation characteristic of the metal 

 in the metal itself, the coefficient of transformation of one 

 into the other type of radiation, and the thickness of the 

 absorbing plate. .\ copper radiation may, by transmission 

 through an iron plate, be transformed .so completely as to 

 be almost indistinguishable from pure iron radiation, but 

 it does not then proceed in the direction of propagation 

 of the incident radiation ; it is emitted from the atoms In 

 approximately equal intensity in all directions. 



\\'hat has previously been described as the special power 

 of a homogeneous radiation of penetrating the element 

 emitting it and elements of neighbouring atomic weight 

 (Phil. Mag., September, 1907, p. 408) may be more precisely 

 stated thus : — A radiation wliich is more absorbable, equally 

 absorbable, or only slightly more penetrating to most sub- 

 stances than the radiation characteristic of the element 

 upon which it Is incident, is absorbed much less than one 

 of more penetrating type. It also produces little or none 

 of the characteristic secondary radiation which is produced 

 by the more penetrating radiation. 



The special power of an ordinary heterogeneous primary 

 radiation after transmission through an absorbing sub- 

 stance of penetrating further layers of that substance is 

 due to two causes — (i) the special absorption of those 

 radiations capable of stimulating a homogeneous secondary 

 radiation, (2) the superposition on the primary radiation 

 of tb.->t secondary radiation. 



A full account of these experiments and a discussion 

 of the results will be published shortly. C. G. Barkla. 



Liverpool, July 8. C. A. Sadler. 



