NATURE 



[May 7, 1903 



The general conclusion is that geometry is largely 

 based on the results of experience. M. de Freycinet's 

 book should prove of great interest to all who devote 

 attention to the teaching of geometry. 



Etude des Phdnomknes volcaniques : Tretnblements 

 de Terre — Eruptions volcaniques — Le Cataclysme de 

 la Martinique, 1902. Par Francois .Miron. Pp. 

 viii + 320. (Paris : Ch. B^ranger, 1903.) 



The ground which this little work is intended to cover 

 is so vast that it is impossible for the author to deal 

 with any part of the subject in an adequate manner. 

 Seismology is dismissed in twenty-seven pages, which 

 serve only to give a most misleading impression of the 

 present state of our knowledge of that science. The 

 ninety-nine pages devoted to volcanic eruptions furnish 

 only a short sketch of the subject, such as may be found 

 in any treatise on geology, though here and there 

 matters not ordinarily treated of in text-books may be 

 met with, such as Fouqu^'s method of collecting gas 

 at fumaroles. The thirty-eight pages devoted to the 

 causes of vulcanism contain summary statements 

 of the views of de Lapparent, Fouqu^, Stanislas 

 Meunier, Gautier and others, the author giving greatest 

 weight to astronomical causes as possibly determining 

 volcanic outbursts ! To the phenomena following 

 volcanic eruptions sixteen pages are devoted, while an 

 account of the principal volcanoes of the globe occupies 

 forty-two pages. The description of the Martinique 

 and St. Vincent eruptions has, however, seventy pages 

 devoted to it, and the work concludes with chapters 

 in which vulcanism and the riches of the globe are 

 discussed, such matters as mineral veins, thermal 

 springs, and the occurrence of petroleum being hastily 

 passed in review. 



It is difficult to understand what useful purpose a 

 compilation of this kind can serve, but, as the author 

 says in his preface, general attention has been attracted 

 by the catastrophe of St. Pierre, and there seems to 

 be a demand for some kind of popular information on 

 the subject. The supply possibly meets the demand, 

 but both are probably ephemeral. 



Experiments with Vacuum Tubes. By Sir D. L. 

 Salomons, Bart. Pp. vii + 49. (London : Whit'taker 

 and Co., 1903.) Price 25. 



Given a well-equipped physical laboratory and an ex- 

 pert glass blower as assistant, one could pass many 

 a pleasant hour In repeating the experiments described 

 in this little book. The phenomena exhibited by 

 vacuum tubes are perhaps the most fascinating that 

 electrical science can show; they possess a rare and 

 peculiar beauty which, like that of the rainbow or the 

 Aurora, appeals to both the aesthetic and the scientific 

 senses. Sir David Salomons describes how tubes may 

 be constructed to produce certain definite results in the 

 arrangement of striae and so forth, and many of the 

 designs give evidence of painstaking ingenuity. A 

 number of experiments with tubes and magnets are 

 also described, some of which serve to illustrate well 

 the mutual action of electric currents and magnetic 

 fields. The author does not deal with those phenomena 

 which. In the hands of Sir W. Crookes, J. J. Thomson 

 and others, have led in recent years to results of such 

 Importance ; indeed, the theoretical explanations which 

 are given as a running commentary on the experiments 

 seem rather to show a lack bf appreciation of the 

 essential facts which have added such interest to the 

 behaviour of the electric discharge In high vacua, and 

 have raised the vacuum tube from the position of a 

 scientific toy to that of a powerful Instrument of re- 

 search. M. S. 



NO. 1749, VOL. 68] 



LETTERS TO THE EDITOR. 



The Editor docs 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 anonymous communications.] 



Energy Emitted by Radio-active Bodies. 



Prof. J. J. Thomson's interesting article in last week's 

 Nature raises the question of how long the emission of 

 energy by radium may be expected to continue. I think 

 in this connection that it would be of great importance to 

 determine, if possible, whether radium, as contained in 

 pitchblende, emits as much energy as the same amount of 

 the material in the form of an artificially concentrated pro- 

 duct. The mineral must be supposed to have been in exist- 

 ence, in its present condition, for a period of time com- 

 parable with the age of the earth — perhaps 50 million years. 

 It is certainly more likely to have lost than gained activity 

 during that time. We may therefore reasonably assume 

 that it has been liberating energy at not less than its pre- 

 sent rate for 50 million years. A determination of the 

 amount of energy thus emitted would carry us much further 

 than the most careful and protracted observations on 

 powerful radium preparations. 



Such a measurement would, no doubt, be difficult, but 

 not, I think, altogether impracticable. A very large block 

 of pitchblende might be used, and a thermocouple inserted 

 in the centre of it. Something might be gained by careful 

 heat insulation of the block. 



A rough calculation will show the rise of temperature to 

 be expected. 



Consider an infinite slab of pitchblende bounded by two 

 plane faces, the axis of x being perpendicular to these 

 faces. Take an elementary slice, of thickness Sx, at distance 

 X from the face, and bounded by planes parallel to it. 



JQg 



The outflow of heat per square cm. from this slice is - ^ — Sx, 



dx- 

 where k is the thermal conductivity, and C the temperature. 



When a steady state has been reached, this must equal 

 the rate of generation of heat in the slice per square cm. = 

 qZx suppose. 



Thus 



- k~lx = qlx, 

 ax'' 



b). 



dx^ k 



and by integration Q — ~ 1 (x' -J- ax - 



2 k 



If the faces of the slab are maintained at 0° C, and if 

 the slab is i metre thick, we have 



/0= o when x = o, 

 yQ= o when x — 100. 

 Thus a - - 100, <5 = o. 



and 



{x - 100) 



We may take for k the value 0005, which is a rough 

 general average for the conductivity of rocks. 



It was found by Curie that i gram of radium emitted 100 

 calories per hour. If we suppose that the density of the 

 radium is 3, and that pitchblende contains one part of it 

 in 100,000 by volume, then, if the pitchblende is as active 

 as one would expect from the proportion of radium con- 

 tained, we should have 



We can now calculate the temperature to be expected at 

 any point of the slab. In the middle, where ^^^so, we find 



d = i nearly. 



So that the middle of the slab would be \° hotter than the 

 faces. 



In practice the difference of temperature available would 

 be less, since the block used would not take the form of 

 an infinite slab. But still, the effect would probably be 

 measurable. R. J. Strutt. 



