342 



NA TURE 



[February 9, 1905 



large proportion of its original activity. On testing it, the 

 activity was found to have reached a small and nearly con- 

 stant value. Rough observations, however, which I had 

 made from time to time indicated that the rate of decay 

 of this polonium was certainly not very different from that 

 of radium E. More accurate experiments will be required 

 to settle the question definitely, but I think there is little 

 doubt but that their rates of decay will be found to be 

 the same. 



Polonium, radio-tellurium, and radium E have very 

 similar radio-active and chemical properties. Each gives out 

 only o rays, and each is deposited on a bismuth plate placed 

 in the active solution. The probable identity of their rates 

 of decay, taken into conjunction with the similarity of 

 their radiations and chemical properties, shows that the radio- 

 active constituent present is in each case the same. We may 

 thus conclude that the active substance present in polonium 

 and radio-tellurium is a decomposition product of radium 

 and is the si.xth (or, as we shall see later, probably the 

 seventh) member of the radium family. 



The main objection, in the past, against the identity of 

 polonium and radio-tellurium has rested on the statement 

 of Marckwald that a very active preparation of his sub- 

 stance did not lose its activity to an appreciable extent in 

 six months. Unless very special methods were employed, 

 it would be difficult to determine with accuracy the varia- 

 tion of the activity for such very active material. The 

 specimen of radio-tellurium obtained both by Meyer and 

 Schweidler and by myself undoubtedly does lose its activity 

 fairly rapidly. 



I have recently examined more carefully the product 

 radium D, and have found strong evidence that it is not a 

 single product, but contains two distinct substances. The 

 parent product, radium D, does not give out rays at all, 

 but changes into a substance which gives out only ;3 rays, 

 and is half transformed in about six days. Unless 

 observations are made on the product radium D shortly 

 after its separation, this rapid change is likely to escape 

 detection. The work on this subject is still in progress, but 

 the evidence at present obtained indicates that the active 

 deposit from the emanation, after passing through the three 

 rapid stages, represented by radium A, B, and C, is trans- 

 formed into a " rayless " product D, which changes ex- 

 tremely slowly. D continuously produces from itself another 

 substance — which may for the time be termed D, — which 

 is transformed in the course of a few weeks and emits only 

 ;8 rays. This product D, gives rise to E (polonium). 



Since the activity of D, reaches a maximum value a few 

 weeks after the production of D, and will then decay at the 

 same rate as D, the conclusion, previously arrived at, viz., 

 that D is half transformed in about forty years, still holds 

 good. 



The view that radium D is the active constituent present 

 in the so-called radio-lead of Hofmann has been very 

 strongly supported by some experimental results recently 

 obtained by Hofmann, Gonder and Wblfl (Annal. der Physik, 

 vol. .XV., 3, 1904). 



They found that preparations of radio-lead continuously 

 produced an a ray product, which could be separated on a 

 bismuth plate. This active product is probably radium E, 

 for they found it lost a large proportion of its activity in 

 one year. They found, in addition, that by certain chemical 

 methods another distinct product could be separated which 

 gave out only $ rays, and lost much of its activity in 

 six weeks. This substance is probably the new radium 

 product D| already referred to. 



Debierne recently concluded that radio-lead and polonium 

 were identical, and proposed that the name radio-lead 

 should be dropped in favour of polonium. In the light of 

 the above results, this position is not tenable. There is no 

 doubt that the preparation of radio-lead in my possession, 

 and also that experimented on by Hofmann, contains a 

 distinct substance which, as the parent of polonium, has 

 certainly as much right to a name as its offspring. The 

 radio-active substance in " radio-lead " has no more con- 

 nection with lead than Marckwald's active matter " radio- 

 tellurium " has with tellurium. The names both arose 

 because the active matter was initially found associated 

 with these substances. 



In order to avoid confusion, I have called the new radium 

 product " radium D,." If no further intermediate products 



NO. I 84 I, VOL. 71] 



of radium are brought to light, it would be simpler to call 

 it radium E and to call the o ray product (polonium) 

 radium F. E. Rutherford. 



McGill University, Montreal, January 24. 



Indian and South A'frican Rainfalls, 1892-1902. 



-Mr. J. R. Sutton, of Kimberley, rendered a signal 

 service to South African meteorology in his " Introduction 

 to the Study of South .\frican Rainfall " {Trans. S.A. 

 Philosophical Soc, December, 1903), but when he states 

 that south-east winds are rare on the south-east coast of 

 South Africa, and that the rainfall of the greater part of 

 the tableland and south-east coast comes from some northern 

 direction (Nature, November 3, 1904), it is difficult to 

 follow his conclusions. jMost, if not all, of those who 

 have studied South African .rainfall will, I think, agree 

 with me that the facts do not bear this interpretation. 

 Least of all is it the case that there has been nothing that 

 can properly be called a drought, in the sense of Sir J. 

 Eliot's address, within the past fifteen years in South 

 Africa. In all the summer rainfall areas of South Africa, 

 viz., over the bulk of the subcontinent, drought has pre- 

 vailed during recent years, and in some localities it has 

 been terribly severe. 



During twenty years I have travelled over every part of 

 South Africa except the desert areas, and I have resided 

 continuously in those parts where there is most rain and 

 forest. I have heard the rain and its mode of arrival 

 discussed in every locality and from every point of view, 

 and these facts have convinced me that the summer rains 

 have their origin in the moist winds from the Indian 

 Ocean. The precipitation of the moisture contained in 

 these humid air currents is caused by barometric depressions 

 with normal cyclonic wind circulation, and it is the winds 

 proper to these depressions that give the appearance of 

 rains coming from the north, north-west, west, &c. 



The following gives a brief account of the various storm 

 types. In Cape Colony storms travel from west to east 

 at all times of the year. .^3 one would expect, they are 

 more regular and better developed in the south than in 

 the north, and in Rhodesia than in the Northern Transvaal. 

 In the north during summer they may be replaced by 

 westward travelling tropical storms. Usually it is the 

 secondary with its thunderstorms, a whirl within a whirl, 

 which precipitates the greater amount of moisture. In 

 the southern portion of the subcontinent these storms in 

 most cases pass across from west to east with their centres 

 to the south, and thus their wind circulation shows at first 

 winds from the north and north-west, then from the west 

 and south-west, and finally from the south and south-east. 

 In summer, when the south-east trade blows on to the 

 subcontinent with a monsoon effect, the wind remains 

 longer in the south-east quarter, and heavy rains come 

 frequently from the south-east or the south-west quarter. 

 The portion of the barometric depression and its accom- 

 panying circulation which brings the wind will depend on 

 the position of the locality, but I have never known the 

 facts not to conform more or less closely to this type of 

 wind circulation. A range of mountains across the south- 

 east rain-producing wind will, of course, increase the 

 precipitation, and when once rain has started in the south- 

 east quarter it will often continue for days with a steady 

 south-east wind blowing like a south-west monsoon wind 

 in India. All this takes place on the eastern side of South 

 Africa. The rain is greatest in amount where the east 

 wind from the Indian Ocean first strikes the highest 

 eastern land, and the rain gradually decreases in amount 

 until the western deserts are reached. It is generally the 

 north-west wind which starts the precipitation, but it is 

 quite certain notwithstanding that the humid currents do 

 not come from the north-west. If, as Mr. Sutton has 

 suggested, the high upper current of the north-west anti- 

 trade were the source of South .'\frican rains, then it would 

 be natural to suppose that the rains would be best de- 

 veloped on the north and western sides of South Africa, 

 which is ex.actly the reverse of what actually takes place. 



South .Africa lies on the border of the south-east trade 

 area. In summer South Africa, from Cape Town to the 

 Zambezi, comes entirely under the influence of the south- 

 east trade winds ; but in winter the southern portion of 



