June i6, 1904] 



NA TURE 



151 



The last critical list of British beetles, by Sharp and 

 Fowler, was published eleven years ago, and we 

 heartily recommend the present list to British entom- 

 ologists. 



A Preliminary Course of Practical Physics. By C. E. 



Ashford, M.A. Pp. 4S. (London : Edward Arnold, 



1904.) Price IX. 6d. 

 This little book on practical physics is of a kind 

 familiar to teachers of the subject. The experiments 

 are simple and well within the power of schoolboys, 

 but so far as we have examined them they differ little 

 from those to be found in well known books. Indeed, 

 in his preface the author says it is impossible adequately 

 to acknowledge the debt " to those from whose books 

 many of the experiments have been derived." But 

 though the book contains much in common with 

 previously published first courses of practical physics, 

 the author has compiled a logical and useful manual 

 of experiments which will serve to introduce boys to 

 the study of physical science. The volume may be 

 recommended to the attention of teachers deciding upon 

 a book to place in the hands of their pupils. 



LETTERS TO THE EDITUR. 

 [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 

 fnaniiscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



On the Radio-activity of Natural Gas. 



I.\ a paper by Mr. E. F. Burton, recently published in 

 the University of Toronto Studies, Physical Science Series, 

 an account is given of some e.xperiments with a highly 

 radio-active gas obtained from crude petroleum. In this 

 investigation it was found that air drawn through crude 

 petroleum became charged with a radio-active emanation 

 which, from the rate at which its activity decayed and 

 from the nature of the induced radio-activity which it 

 produced, the author concluded to be an emanation from 

 radium. 



The present writer has extended this investigation to an 

 fxamination of the natural gas from different wells in 

 western Ontario. The gas from every well examined, 

 which included those in the Welland district, in the neigh- 

 bourhood of Niagara Falls, as well as those near the city 

 of Brantford, was found to be charged with a radio-active 

 emanation. The activity of this emanation in all the gases 

 tested was found to decay or die out to one-half its original 

 intensity in about three days, and the intensity of the 

 induced radio-activity which it produced died down to one- 

 half value in about forty minutes. 



The wells examined varied in their depths, but the amount 

 of active emanation present was found to be practically the 

 same in all wells coming from the same horizon. In the 

 Welland district, the gas from those wells which had their 

 source in the stratum known as the Niagara formation, and 

 which were about 500 feet deep, possessed the highest 

 initial conductivity. On an arbitrary scale this conductivity 

 is represented by about 2000. 



The gas of those wells which had their source in the 

 Clinton limestone, 750 feet deep, possessed an initial con- 

 ductivity of about 300 on the same scale, while that from 

 wells coming from the Medina formation, about 900 feet 

 deep, gave an initial conductivity of about 1200. One 

 well, which had its source in the Trenton limestone, and 

 had a depth of about 3000 feet, possessed an initial con- 

 ductivity of about 200. The highest conductivity obtained 

 in the investigation was that of the gas from a well near 

 the city of Brantford, the conductivity in this case being 

 about 9000. An investigation of this gas showed that, 

 under the action of the emanation with which it was 

 charged, there was produced, at normal pressure, about 

 15,000 ions per second in each cubic centimetre of its mass. 



A test made on the conductivity of ordinary air, confined 



NO. 1807, VOL. 70] 



at atmospheric pressure in the receiver used in making the 

 measurements on the conductivity of the different samples 

 of natural gas, showed a production of 32 ions per cubic 

 centimetre per second. J. C. McLenn.-vn. 



University of Toronto, May 28. 



The Source of Radio-active Energy. 



In Nature of June 2, Mr. Jeans brings forward the view 

 that the energy manifested in radio-active processes is derived 

 from the coalescence of positive and negative ions, thus 

 involving an annihilation of matter. For some time it has- 

 seemed to me that some such fundamental change is needed 

 to account for the observed phenomena, and I therefore 

 venture to submit some general and numerical considerations- 

 bearing on this view. 



Mr. Jeans is inclined (as I understand) to attribute the 

 beginnings of the process to a change of type in advancing 

 aethereal waves, arising from a lack of strict linearity in the 

 equations of the electromagnetic field. It may be pointed 

 out, however, that whether or not the circumstances of 

 jEthereal wave-propagation are strictly expressible by linear 

 equations, there is a universal tendency towards loss of 

 kinetic energy in orbitally moving systems of electrons. 

 Unless the orbital periods are very long compared with the- 

 time taken by radiation to traverse the assemblage, there 

 must be appreciable radiation of energy, and it is thus a 

 necessary condition of permanence or quasi-permanence that 

 the orbital velocities should be very small compared with the 

 velocity of light. This view is confirmed by numerical con- 

 sideration of simple cases in which the orbits are assumed 

 to be of atomic dimensions; it is also borne out by the 

 general optical properties of matter. 



It should be remarked that as energy is dissipated and 

 orbits become contracted, with corresponding rise of veloci- 

 ties, the total effective radiation will become more and more 

 intense, so that conceivably very little time may be occupied, 

 in the transition from a quasi-permanent motion to a state- 

 of collapse and disintegration ; indeed, once the orbital- 

 motions have begun to give out perceptible radiation, the 

 life of the system must be excessively short. 



Thus, whether we look for the main source of radio- 

 active energy in enormous orbital velocities due to intra- 

 atomic rearrangement, or in the constitutive electrostatic 

 energy of individual electrons set free by mutual annihil- 

 ations, the conditions favourable to radio-activity in any 

 given atom must be confined to a momentary phase-^ 

 momentary, that is, as measured by ordinary standards- 

 It is not a long step from this conclusion to an exponential 

 law of decay of radio-active matter. 



If we adopt provisionally Dr. H. A. Wilson's very interest- 

 ing suggestion (Nature, June 2) that, the positive and 

 negative electrons having numerically equal charges, the 

 greater mass of the positive electron is due to its smaller 

 diameter, it follows that any isolated electron has electro- 

 static energy = 5"J. 3V", where m is the mass of the electron 

 (when moving slowly) and V is the velocity of light. In 

 other words, when matter of_ mass M is annihilated, 

 energy = 5M.3V- is set free — initially as an electromagnetic 

 pulse of great intensity. A further assumption involved in 

 this estimate is the validity of the ordinary electrostatic-field' 

 relations for such enormous intensities as obtain in the 

 neigiibourhood of an electron. 



If annihilation of matter furnishes the energy of radio- 

 activity, it follows from our estimate that, in the case of 

 radium, the coalescence of one pair of electrons causes the 

 break-up of a large number of radium atoms (something 

 of the order of one hundred), otherwise the total energy 

 emitted by radium would be much greater than that which 

 has been observed by Curie and Laborde. 



If the assumption in italics above is very wide of the 

 mark (which is conceivable), our estimate of the energy of 

 annihilation is probably in excess. 



It may be supposed that some neighbouring atoms, which 

 are not actually broken up by the pulse arising from a pair 

 of coalescing electrons, receive a sufficient access of kinetic 

 energy to prolong their existence. " Metabolons " of short 

 average life may be conceived of as consisting of assemblages 

 the orbital motions of which are especially liable to be 

 damped out rapidly by radiation of energy. 



Cambridge, June 9. C. V. Burton. 



