Al'KIL 2, I903J 



NA TURE 



5ii 



determining factor in the weather at any place. The 

 unfailing agreement shown by these calculations 

 arouses suspicion. On closer examination, we find 

 that the signs of the corrections vary quite arbitrarily, 

 while at least five different methods of correcting for 

 declination occur in the text. Results based on such 

 foundations cannot inspire much confidence even 

 though a fair agreement between predictions and Green- 

 wich^ records is claimed. The more obvious method 

 of exhibiting the similarity of meteorological condi- 

 tions under similar astronomical conditions by compar- 

 ing the corresponding isobaric charts does not appear 

 to have occurred to Mr. Clements. We commend this 

 method to the attention of those who have leisure to 

 devote to a detailed examination of a mode of dealing 

 with meteorology that recurs from time to time. 



Bis mi's Kmic der Welti Astronomische Causerien. 



Third Edition. Pp. 212. By Prof. F. J. Stud- 



nicka. (Prague : Published by the Author, 1903.) 

 Tins book, which was dedicated to the celebration of 

 Christian Doppler's hundredth birthday, has reached 

 a third edition. It is written in the form of a conver- 

 sation among men of various professions meeting 

 socially together every day with the intention of con- 

 veying in popular language many astronomical ideas. 

 " To the end of the Universe " is the subject o! a dream 

 which one of the members of this convivial party, 

 Carpenter bv name and astronomer by profession, had 

 dreamt, and the narrative is his account of this dream 

 to his companions, subject, of course, to many inter- 

 ruptions by one or other of them seeking more informa- 

 tion or more detailed explanation. 



The author has quite succeeded in his object, and 

 the book will be found to contain an admirable ex- 

 position of some of the more general astronomical 

 topics. Being printed in large and Roman type, it 

 should find many readers in this country. 

 Die radioactiven Stoffe nach dem gegenwartigen 



Stande dcr wissenschajilichen Erkenntnis. By Karl 



Hofmann'. Pp. 54. (Leipzig: Ambrosius Barth, 



1903.) Price 1.60 marks. 

 This book contains a concise account of the discovery 

 and subsequent investigation of the radio-active 

 elements bv Becquerel, the Curies, Rutherford and 

 others. It is written mainly from a chemical stand- 

 point, and many of the effects which have been 

 accurately measured, especially by Rutherford, are re- 

 ferred to as though they had been merely observed 

 and not measured. For example, Rutherford has 

 shown that the radio-activity of thorium-X dies awa) 

 with time according to the formula e~ kt , where t is 

 the time and k a constant, but Hofmann merely men- 

 tions that the activity dies away. The book contains 

 references to the original papers published before the 

 latter half of 1902, and should prove useful to those 

 wishing lo study the subject. H. A. \Y. 



Carnei de Notes d'un Voyageur en France. Par A. 



('. Poire. Pp. viii + 169. (London: Macmillan 



and Co., Ltd., 1903.) Price 15. 6d. 

 M. Poire intends this note-book for boys who will in 

 the future be merchants and manufacturers. The 

 piovinces and important commercial centres of France 

 are described only from industrial, commercial and 

 agricultural points of view; historical, administrative 

 and geographical details have been omitted as being 

 unnecessary for the particular class ol student for 

 whom the book is written. 



At the bottom of each page is a vocabulary of diffi- 

 cult or unusual French words. By the time the student 

 has worked through the volume he will not only have 

 much improved his knowledge of French, but have ac- 

 quired considerable acquaintance with the character- 

 istics of different parts of France. 



NO. 1744, VOL. 67] 



LETTERS TO THE EDITOR. 

 [The Editor docs not hold himself responsible for opinions 

 expressed by Ins correspondents. Neither can lie 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 0/ anonymous communications.] 



Radium Emission. 



Concerning the recently discovered heat emission from 

 radium, it is perhaps worth noting that it appears to be 

 connected with, and is probably an immediate consequence 

 of, the remarkable observation 'by Rutherford that radium 

 emits massive positively-charged particles, which are prob- 

 ably atoms, with a velocity comparable to one-tenth of the 

 speed of light (see Phil. M~ag., February. 1S03). 



Because it is easy to reckon that the emission of a million 

 heavy atoms per second, which is a small quantity barely 

 weighable in a moderate time such as a few weeks 

 (being about the twentieth part of a milligramme per 

 century), with a speed equal to one-tenth that of light, 

 would represent an amount of energy equal to one thousand 

 ergs per second ; that is to say, would correspond to heat 

 enough to melt a milligramme of ice every hour. And in- 

 asmuch as these atoms are not at all of a penetrating kind, 

 but are easily stopped by obstacles, they would most of them 

 be stopped by a small thickness of air, and their energy 

 would be thus chiefly expended in the immediate proximity 

 of the source, which source would thereby tend to be kept 

 warm. 



It would appear on this view as if by enclosing a bit of 

 radium in a small chamber formed of massively obstructing 

 non-conducting walls that it could be made quite hot ;' pro- 

 vided always that the assumed necessary stimulus, or ex- 

 ternal supply of molecular energy, could get at it un- 

 interruptedly. 



If, in the open, the rate of escape ol heal were such that 

 on the average it accumulated for one minute before 

 escaping, the temperature of source and ambient air, with 

 an assumed heat-capacity equal to that of one milligramme 

 of water, would amount to one and a half degrees 1 eni i- 

 grade. Oliver Lodge. 



March 2S. 



Radio activity of Ordinary Materials. 



In connection with the article by Mr. Strutt on this sub- 

 ject in Nature of February 19, and the letter by Prut. 

 J. J. Thomson in the following week, it may he of interest 

 to mention some work along similar lines that has been 

 in progress in the McGill Physical Laboratory since Sep- 

 tember last. 



At the same meeting of the American Physical Society in 

 Washington last December, at which the interesting paper 

 of Dr. MacLennan and Mr. Burton, referred to by Prof. 

 J. J. Thomson, was presented, an account was given by 

 Mr. H. L. Cooke and myself of some results showing that 

 a very penetrating radiation was given off from the walls 

 of buildings and from the surface of the earth itself. 



The primary object of this investigation was to see if the 

 natural ionisation of air observed in closed vessels was due, 

 in part at least, to an external radiation which passed 

 through the walls of the vessel. For this purpose the rate 

 of discharge of a gold leaf electroscope in a brass vessel el 

 about 1 litre capacity was observed. When the closed vessel 

 was surrounded bv thick screens of lead and iron, the rate 

 of discharge was reduced about 30 per cent. A similar 

 effect was observed when the electroscope was immersed 

 in a deep water tank. No further reduction of the discharge 

 was observed when the electroscope was surrounded with five 

 tons of lead. These results showed conclusively that about 

 30 per cent, of the ionisation in closed vessels was due to an 

 external radiation of great penetrating power which passed 

 readily through 1 cm. of lead. In a brass electroscope, sur- 

 rounded by thick screens, the number of ions produced was 

 reduced to as low as five per c.c. per second. In the course 

 of these experiments, Mr. Cooke observed that a layer ot 

 brick round the electroscope increased the rate of discharge 

 instead of diminishing it, pointing to the conclusion that 

 the brick was itself radio-active. Mr. Cooke has extended 

 these observations, using cylinders of different metals placed 

 inside the electroscope, with results of a similar character 

 to those already recorded bv Mr. Strutt in his article. 



