572 



NA TURE 



[October 15, 1903 



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 anonymous communications.] 



Radium and the Sun's Heat. 



In your last week's issue Mr. Hardy directs attention to 



the fact that no Becquerel rays can be detected from the 



sun, and regards this as an objection to the view that the 



solar heat may be accounted for by the presence of radium. 



Let us attempt to calculate the effect to be expected if 

 the sun's heat were due to this cause. 



In doing this, we may assume that the sun contains 36 

 grams of radium per cubic metre. This was the amount 

 which Mr. W. E. Wilson gave in Nature of July 9 as 

 required to emit the observed amount of heat. Experiment 

 shows that when the Becquerel radiation has to pass 

 through lead screens of thickness i cm. or more, the radi- 

 ation transmitted is practically all of the 7 variety. This 

 is cut down to half its value by 8 cm. of aluminium, and 

 in the case of other substances by strata of equal mass per 

 unit area. Now the earth's atmosphere constitutes a 

 stratum far more absorbent than i cm. of lead. We need, 

 therefore, only consider the 7 rays, for if these cannot be 

 detected, it is certain that the a and fi rays cannot. 



For the sake of simplicity of calculation, we shall treat 

 the sun as a cube, with its side equal to the diameter of 

 the real sun, and so placed that the normal to one face, 

 which passes through the centre, shall also pass through 

 the earth. This will be for all practical purposes near 

 enough to the truth. 



Let a be the side of the cube, q the quantity of radium 

 pe*- c.c, and X the coefficient of absorption of the radiation. 

 Then, from an elementary slice, thickness dx, and distance 

 X from the face, the intensity of radiation at a distant point 

 will be 



if the radiation due to i gram of pure radium at the same 

 (great) distance be taken as unity. 



The radiation due to the entire mass will be 



n 



Now a=i-4Xio^' cm.; q, from Mr. Wilson's estimate = 

 3.6X I0-^ 



Assuming that the coefficient of absorption is proportional 

 to the density, and taking the sun's density as 1/7, and 

 the value of X for aluminium as 0086, the value of X for 

 the sun comes out 00046. Substituting these values, we 

 find that the effect of the sun is equivalent to that of 

 1-53x10^' grs. of radium at the same distance, assuming 

 this radium to be spread out into a thin layer, so that all 

 the radiation can escape without undergoing absorption in 

 the mass. 



Now I have found that the 7 radiation from 10 milligrams 

 of radium bromide can barely be detected by the electrical 

 method, where 10 cm. of lead intervene between it and the 

 testing vessel. To decide whether the solar rays would be 

 detectable, we must compare their expected effect after 

 enfeeblement by distance, and by the absorption of the atmo- 

 sphere, with this. 



The distance of the sun is 1.5X10'^ times greater than 

 the distance of the radium from the testing apparatus, so 

 that, apart from the atmospheric absorption, the effect of 



J, ex 10" 

 the sun would be equivalent to that of . - ' .g gi' ^"^ 



6-7x10-' grams of radium, 10 cm. from the apparatus. 

 This is less than one-thousandth part of the radium used 

 in the experiment cited, and the solar radiation, instead of 

 passing through only 10 cm. of lead, would have to pass 

 through the atmosphere, equal in mass to 32 feet of water, 

 or about 89 cm. of lead. This would, of course, reduce it 

 many million times further. So that, even if all the sun's 

 heat were due to radium, there does not appear to be the 

 smallest possibility that the Becquerel radiation from it 

 could ever be detected at the earth's surface. 



R. J. Strutt. 

 NO. 1772, VOL. 68] 



Referring to Mr. Hardy's experiment described in his 

 letter in Nature, October 8, it is easy to show that what- 

 ever the intensity of radio-activity might be at the surface 

 of the sun, by mere surface ratios and assuming no absorp- 

 tion its activity per unit area at the distance of the earth 

 must fall to about one forty-thousandth part. Now, if the 

 sun were composed of solid radium bromide, the radiation 

 reaching Mr. Hardy's indicator from the sun will be only 

 about one-thousandth part of that derived from a sphere 

 of radium bromide three millimetres in diameter and twenty 

 millimetres distant from the indicator : the probable con- 

 ditions of Mr. Hardy's experiment. 



In the e.xperiment one centimetre thickness of lead is 

 interposed. The earth's atmosphere is equivalent in mass 

 to 76 cm, of mercury. This supposes no absorption from, 

 possibly, some thousands of miles of solar atmosphere. 

 Moreover, we assume in the comparison a sun of solid 

 radium bromide. It would appear, however, that a very 

 small percentage of this body in the materials of the sun 

 would suffice to account for many millions of years of solar 

 heat. 



The absence of /3 and 7 radiations at the earth's surface 

 is, therefore, not a weighty argument against the presence 

 of radium in the sun. 



The arguments in favour of supposing that this element 

 exists in the sun are: — (i) The presence of radium on the 

 earth ; (2) the high atomic weight of radium ; (3) the 

 presence of helium in the sun ; (4) Arrhenius's theory of the 

 Aurora Borealis ; (5) the fact that the estimate of the dura- 

 tion of solar heat from the dynamical source appears to run 

 counter to geological data. J. Joly. 



Trinity College, Dublin, October 10. 



Cambridge in the Olfl World and in the New, j 



One of the most striking features of the universities of 

 the United States is the wealth of their endowment. 

 During the writer's visit to Cambridge, Massachusetts, for 

 example. Harvard University was successfully collecting 

 large sums towards a new buildinj; for philosophy in 

 memory of Emerson, and within the last few months has 

 been promised two million dollars by two millionaires to- 

 wards her new medical school. • 



Reasons for such well-known munificence of Americans 

 towards their universities are not hard to find. Pauperism 

 is an almost negligible quantity in America, so that the 

 money, which drains away on this side in charity, finds 

 ai outlet there in the advancement of education and re- 

 search. Primogeniture, again, is contrary to American 

 ideals. While the newly-made English millionaire thinks 

 it his duty to sink a considerable part of his fortune in 

 buying and maintaining a family estate for his son and 

 heir, the American more often divides his property equally 

 between his children, and feels at greater liberty to dispose 

 of much of it in his lifetime as he pleases, for he is willing 

 that the uphill life he has lived himself shall be lived again 

 by his descendants. The absence of inherited titles in 

 America tends, of course, towards the same end. Many of 

 the younger universities, too, are in districts where huge 

 fortunes have been rapidly made and civic pride runs high, 

 producing numerous benefactions in the cause of local in- 

 stitutions. But although all these are reasons, none of 

 them is sufficient to explain the situation satisfactorily. To 

 find the true cause, we must enter into the differences in 

 life and education between the older English and American 

 universities. 



The average English youth, passing from public school 

 to Oxford or Cambridge, intends to make his living by 

 some profession, perhaps as minister, teacher, barrister, or 

 physician ; relatively seldom has he sufficient to live upon 

 without further exertion. He spends his three or four 

 years in one of the seventeen or more colleges from which 

 he has to choose, and his college becomes the centre of 

 his social life. Probably there he makes his greatest friend- 

 ships ; certainly the number of men he knows outside his 

 own college is comparatively small. In eights, elevens, or 

 fifteens, the various colleges are pitted against one another. 

 Nor, indeed, is inter-collegiate competition confined to 

 athletics. Each college is continually struggling against 

 the rest to secure the most promising boys from the public 

 schools, and to acquire the greatest number of university 

 distinctions. Each has to maintain a more or less separate 



