June i6, 1904J 



NA rURE 



161 



sky at the time when the photographs were taken. Prof. 

 Campbell suggests that cameras similar to those used by 

 Perrine should be used in Labrador, Spain, Tunis, and 

 Egypt. He also insists upon the necessity for setting up 

 coronagraphs at each of these widely separated stations in 

 order to determine whether or not any real changes take 

 place in the corona during the eclipse. 



The determination of the correct wave-length of the chief 

 corona line is also suggested as being of great importance. 

 Finally, he urges upon observers the vital importance of 

 thoroughly testing all their instruments before leaving 

 home, and of allowing themselves plentv of time to make 

 the final adjustments whilst in the eclipse camp. 



AcTi'AL DiST.iNXEs BETWEEN -St.ars. — By simple trigono- 

 metrical calculations, Mr. J. E. Gore has deduced some 

 interesting facts regarding the probable actual distances 

 between certain stars the parallaxes of which are known 

 with some degree of certainty. Thus he has determined 

 that Sirius and Procyon are separated by about half the 

 distance between the former star and our own system, 

 therefore Sirius as seen from Procyon would appear as 

 a star of magnitude —308. In the case of 57 and (i Cassio- 

 peia-, the actual distance between them is probably about 

 one-fifteenth their distance from the sun, and their apparent 

 brilliancies would therefore be about 225 times as great as 

 they appear to us. In the case of double stars, these figures 

 become much greater; for e.xample, if we take 61 Cygni, 

 the distance separating the components is about 55 astro- 

 nomical units, and, as they are probably situated at some 

 515,662 astronomical units from the earth, their apparent 

 brightness would be increased about 88 million times, or 

 by igS magnitudes, if seen at the distance which separates 

 them. Similarly, the brightness of each of the components 

 of o Centauri would be increased by ig 7 magnitudes (the 

 Observatory, No. 345). 



T//E SUCCESSION OF CHANGES IN RADIO- 

 ACTIVE BODIES} 



TT has been shown by Rutherford and Soddy that the 

 radio-activity of the radio-elements is always accom- 

 panied by the production of a series of new substances 

 possessing some distinctive physical and chemical proper- 

 ties. These new substances are not produced simul- 

 taneously, but arise in consequence of a succession of 

 changes originating in the radio-elements. The radio- 

 activity of these products is not permanent, but diminishes 

 in most cases, according to an exponential law with the 

 time. Each product has a distinctive rate of decay of 

 activity, w'hich has not. so far, been altered by any physical 

 or chemical agency. The law of decay has been explained 

 on the supposition that the product undergoes change accord- 

 ing to the same law as a mono-molecular change in 

 chemistry. The change occurs in consequence of the ex- 

 pulsion of an a or /3 particle, or both, and the activity of 

 a product is thus a measure of its rate of change. While 

 the products, like the emanations, and UrX, lose their 

 activity according to an exponential law, the matter eman- 

 ation X, which gives rise to the phenomena of excited 

 activity, does not lose its activity according to a simple law. 

 The experiments of Miss Brooks and the author, and of 

 Curie and Danne, have shown that the decay of the excited 

 activity of radium is very complicated, and depends upon 

 the time of exposure to the exciting cause, viz. the eman- 

 ation. The author has shown that the excited activity pro- 

 duced in a body by a short exposure in the presence of the 

 thorium emanation increases at first for a few hours, passes 

 through a maximum value, and then decays with the time 

 according to an exponential law. 



In the paper the curves of decay of excited activity of 

 radium and thorium are given for both short and long ex- 

 posures to the emanations, and it is shown that the law 

 of change of activity with time can be completely explained 

 on the theory that emanation X of thorium and radium is 

 complex, and undergoes a series of successive changes. 



The mathematical theory of successive changes is given 

 in detail, and a comparison is made of the theoretical and 



experimental curves obtained for the variation with time 

 of the excited activity. In the case of thorium, two changes 

 are found to occur in emanation X. The first change is a 

 " rayless " one, i.e. the transformation is not accompanied 

 by the appearance of a, 5, or y rays. The second change 

 gives rise to all three kinds of rays. 



The decay of activity of emanation X of radium depends 

 greatly on whether the o or j3 rays are used as a means of 

 measurements. The curves obtained by the ;3 rays are 

 always identical with those obtained by the 7 rays, showing 

 that the /3 and 7 rays always occur together and in the 

 same proportion. The complicated decay curves obtained 

 for the different types of rays, and for different times of 

 ivposure, can be completely e.xplained on the supposition 

 that there are three rapid successive changes in the matter 

 deposited by the emanation, viz. : — 



(i) .A rapid change, giving rise only to a rays, in which 

 half the matter is transformed in about three minutes. 



(2) A " rayless " change, in which half the matter is 

 transformed in twenty-one minutes. 



(3) .A. change giving rise to a, $ and 7 rays together, in 

 which half the matter is transformed in twenty-eight 

 minutes.' 



.\ similar rayless change is shown to occur in the " eman- 

 ating substance " of Giesel. 



The occurrence of a rayless change in the three radio- 

 active bodies is of considerable interest. Since the change 

 is not accompanied by rays, it can only be detected by its 

 effect in the change or changes which follow. The matter 

 of the rayless change is transformed according to the same 

 law as the other changes. The rayless change may be sup- 

 posed to consist either of a rearrangement of the components 

 of the atom or a disintegration of the atom, in which the 

 products of the disintegration are not set in sufficiently 

 rapid motion to ionise the gas or to affect a photographic 

 plate. The significance of the rayless changes is discussed, 

 and the possibility is pointed out that similar rayless changes 

 may occur in ordinary matter ; for the changes taking place 

 in the radio-active bodies would probably not have been 

 detected if a part of the atom had not been expelled with 

 great velocity. 



The radiations from the different active products have 

 been examined, and it is shown that the ;8 and 7 rays appear 

 only in the last rapid change of each of the radio-elements. 

 The other changes are accompanied by the emission of a 

 particles alone. 



Evidence is given that the last rapid change in uranium, 

 radium, and thorium, which gives rise to and 7 rays, is 

 far more violent and explosive in character than the pre- 

 ceding changes. There is some evidence for supposing that, 

 in addition to the expelled a and /3 particles, more than one 

 substance is produced as a result of the disintegration. 



.\fter the three rapid changes have taken place in eman- 

 ation X of radium, there remains another product, which 

 loses its activity extremely slowly. Madame Curie showed 

 that a body, which had been exposed for some time in the 

 presence of the radium emanation, always manifested a 

 residual activity which did not appreciably diminish in the 

 course of six months. A similar result has been obtained 

 by Giesel. Some experiments are described, in which the 

 niatter of slow decay, deposited on the walls of a glass tube 

 containing the emanation, was dissolved in acid. The 

 active matter was found to emit both a and $ rays, and 

 the latter were present in unusually large proportion. The 

 activity measured by the /3 rays diminished in the course of 

 three months, while the activity measured by the a rays 

 was unaltered. The active matter was complex, for a part 

 which gave out only a rays was removed by placing a 

 bismuth plate in the solution. The radio-active matter 

 deposited on the bismuth is closely allied in chemical and 

 radio-active properties to the active constituent contained 

 in the radio-tellurium of Marckwald. The evidence, as a 

 whole, is strongly in support of the view that the active 

 substance present in radio-tellurium is a disintegration pro- 

 duct of the radium atom. Since the radium emanation is 



1 A sia 



t of the 



X 



1 Bakerian T.eclure delivered at the Royal Society 

 Prof. E. Rutherford, F.R.S. 



NO. 1807, VOL. 70] 



May 19 by 



(/'/,;7. Afng-., February). .A. br 

 results were deduced has been given in 

 bridge University Press). Later, Cur 

 March 14) arrived, in a similar way, at t 



i of the three changes occurring in emanation 

 given in a paper by Rutherford and Barne 



of the theory from which th 

 ny book " Radio-activity "(Cam- 

 ; and Dame (Camptes rendus. 



