November 14, igi2j 



NATURE 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neitner 

 can he undertake to return, or to correspond with 

 '.lie writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken oi anonymous cotnmunications.] 



Radium and Earth History. 



It would appear that radium has landed geologists 

 and biologists in a difficulty greater than that trom 

 which it was hoped it would deliver them. There is 

 radium in the earth, and radium in disintegrating 

 gives out heat. Therefore a once molten globe will 

 cool down more slowly than if it contained no such 

 independent source of heat. Lord Kelvin's calcula- 

 tions were made on the supposition that there was no 

 source of heat except what the earth possessed as a 

 molten globe. Hence we are at liberty to e.\tend the 

 time that has elapsed since the earth became the pos- 

 sible theatre of geological change to 500,000,000, 

 1,000,000,000, or even more, years ago. Radium has 

 given us a blank cheque on the bank of time. 



So far so good. But when the actual calculations 

 were made as to how much the radium known to 

 exist in the outer shell of the earth would effect its 

 cooling, this w^as found to be too great. It would, in 

 fact, raise the temperature of the earth the fraction 

 of a degree annually. 



Two suggestions in the way of explaining the diffi- 

 culty have been made by Prof. Joly in his " Radio- 

 activity and Geology." We do not think that either 

 will bear the test of careful examination. 



It is only the outer shell of the earth that can be 

 examined for radium, and though there appears to be 

 no diminution with depth, there may be less, or none, 

 in the lower parts. If, then, we have to spread the 

 heating effects of the radium of the outer shell over 

 the whole earth, it will obviously be insufficient to 

 raise its temperature. The only possible result of its 

 disintegration w'ill be a retardation of its cooling to an 

 indefinite extent, which is what is wanted. This is 

 the first suggestion. 



The second, admitting that the proportion of radium 

 in the interior may be the same as at the surface, 

 avails itself of the fact that vast masses cf the central 

 earth may be thermally isolated for immense periods 

 of time. The rise in temperature of such parts — due 

 to their radium — need not, then, affect the rocky crust. 

 In the course of prolonged ages, however, such in- 

 ternal reservoirs of heat might, so to speak, over- 

 flow. Great rushes of heat might reduce the outer 

 shell to a molten state, and inaugurate a new geo- 

 logical era. To quote Prof. Joly : — 



" With an interest almost amounting to anxiety, 

 geologists will watch the development of researches 

 which may result in timing the strata and the phases 

 of evolutionary advance ; and may even — going still 

 further back — give us reason to see in the dis- 

 crepancy between denudation and radio-active methods, 

 glimpses of past :eons, beyond that dav of regenera- 

 tion which at once ushered in our era of life, and, for 

 all that went before, was 'a sleep and a forgetting.'" 



But let us look at these interesting suggestions a 

 little more closely. If the radium contents of the 

 outer shell were spent in heating the whole earth — 

 or any considerable portion of it beyond the shell con- 

 taining it — then we might suppose it just sufficient to 

 retard its cooling indefinitely. But as the temperature 

 of the earth increases with depth, we cannot suppose 

 that any of the radium-generated heat of the outer 

 shell passes downwards. It must all be spent in heat- 

 ing its own ?)7a.'x. Therefore, according to calcula- 

 tion, this outer shell should be rising in temperature. 



NO. 2246, VOL. go] 



There seems to be no escape from this conclusion. 

 .A,nd this applies also — and even more forcibly — to the 

 second explanation. For with an interior rising in 

 temperature it is still more ditYicult to imagine any of 

 the radium-generated heat of the outer shell passing 

 downwards. The radium heat of the crust must all be 

 spent on itself. 



Even this does not express the full extent of the 

 difficulty. The theory of the radio-active elements is 

 that they have their periods in which they lose half 

 their substance. The period of radium is 1760 years, 

 and that of uranium 5,000,000,000 years. Now, since 

 w-e know of no source whereby the supply of uranium 

 in the earth is replenished, we must suppose that 

 there w^as twice as much uranium 5000 million years 

 ago as there is to-day. And whatever length of time 

 we go back we must suppose there was more 

 uranium, and hence a greater heating effect, than 

 there is to-day. A molten globe could not begin to 

 cool until the radium contents of its outer shell were 

 less than that of the earth to-day. 



The moon presents another difficulty. Our satellite 

 is generally held to be a bit of the earth thrown off 

 some fifty-six million years ago. It was then molten, 

 and the drag of the tides produced in its molten mass 

 by the earth gradually reduced its rate of rotation. 

 Now it only turns once on its axis in tlie course of a 

 revolution round the earth. The moon's radium has 

 not prevented it reaching a stage of cooling far beyond 

 that of the earth. And yet the moon may be supposed 

 to have had the full proportion of radium known to 

 exist in the outer shell of the earth. Yet it has cooled 

 down from a molten state in fifty-six million years in 

 spite of its radium! And it would appear that the 

 earth has done the same, although it has not reached 

 the same stage. For if the moon was molten when 

 it began its separate existence, so must the earth — 

 which gave it birth — have been. 



.'\nd it would appear that there must be more radium 

 in the sun than in the earth. For helium, the product 

 of the disintegration of radium, was discovered spectro- 

 scopicallv in the sun years before it was known on 

 earth. It must surely, therefore, exist there in much 

 larger quantities. Hence the sun should be getting 

 hotter at a greater rate than the earth. 



The difficulties introduced by radium into earth 

 history are greater than that which it was hoped it 

 would remo\"e. G. W. Bulman. 



The Moon and Poisonous Fish. 



May I ask for a little space in your columns to 

 inquire if any of your readers can give any informa- 

 tion as to the origin of a belief, very widespread in 

 South Africa, that fish exposed to the influence of 

 moonlight becomes poisonous? I have not yet 

 attempted any experiments to test the truth of the 

 statement, nor have I been able to obtain actual 

 evidence of illness or death following the eating of 

 such fish. The belief is very firmly rooted here among 

 all classes of persons, but no one seems able to say 

 how or when it originated, or on what grounds it is 

 based. One very trustw-orthy witness told me that he 

 had accidentallv left some fresh fish out in the moon- 

 light one night, and that it was quite bad in the 

 morning ; he admitted, however, that the closeness of 

 the atmosphere might have occasioned the effect, and, 

 of course, he did not divide the sample so as to keep 

 part of it unexposed. I intend to test the statement 

 experimentally when opportunity offers; meanwhile 

 possibly some of your readers can say whether the 

 belief in this ill-effect of moonlight is found in other 

 localities. E. G. Bryant. 



Grey Institute, Port Elizabeth, Cape Colony, 

 October 7. 



