August 



!9i3] 



NATURE 



5«- 



arguments (in which the limited distribution of the 

 radio-elements is deduced from the earth's tempera- 

 ture-gradient) is made the basis of a revival of 

 Arrhenius's view that radio-active disintegration may 

 be inhibited under the conditions prevailing at great 

 depths (Nati/re, June 26, July 10, and July 17). Thus, 

 in place of the deduction that the amounts of uranium 

 and thorium existing in the earth's interior are 

 negligible, even thermally negligible, is put forward 

 the alternative deduction that these parent elements 

 are not necessarily absent, but only temporarily impo- 

 tent, their output of energy, by which alone we could 

 be aware of them, being inhibited by the enormous 

 pressures to which they are subjected. 



The latter alternative is favoured by the philosophic 

 conception embodied in Le Chatelier's law of reaction, 

 viz. that the internal reactions within a material 

 system are such as will tend to oppose any external 

 influences by which its equilibrium may be disturbed. 

 It would at once be deduced from this "law" that 

 radio-active transformations, implying as they do an 

 immense output of energy in the form of electronic 

 and atomic bombardments, must be inhibited by the 

 application of sufficiently high pressure and tempera- 

 ture. Under high pressures the internal reactions will 

 tend to oppose the pressure and therefore an increase 

 of volume, and similarly under high temperatures 

 the internal reactions will tend against a further rise 

 of temperature. The internal reaction is favoured 

 which results in the absorption of energy from an 

 external source. Hence, radio-active changes, in- 

 volving increase of volume and rise of temperature, 

 would presumably be opposed by the physical condi- 

 tions of the earth's deep interior. 



\s yet, however, we are unable to assert whether 

 this deduction may safely be made to the extent of 

 assuming inhibition. Frankly, it is a step taken in 

 the dark. The law of reaction is known to be a useful 

 -uiil. as regards the tendency of molecular activities; 

 its extension to include the internal activities of atoms 

 has yet to be demonstrated. The evidence from direct 

 experiment is manifestly insufficient, but so far as it 

 goes it betravs on the part of the radio-elements an 

 astonishing disregard for all external influences. For 

 the present, then, the problem stands unsolved, as 

 indeed it must remain until we know more of the 

 internal mechanism of the radio-active atom. 



However, whether one favours radio-active inhibi- 

 tion or independence, the remaining arguments in 

 support of a crustal concentration of radium stand 

 unaffected. The highest concentration of the radio- 

 elements is found in the acid rocks, which contain 

 six times as much radium as the ultra-basic rocks. 

 That is to say, the lighter rocks in their capacity as 

 solvents are capable of carrying a higher proportion 

 of the radio-elements than are the heavier rocks. It 

 is for this reason that uranium, which is the heaviest 

 element of all, has not gravitated to the earth's 

 metallic core, as Dr. Schiller considers would be most 

 probable. Further, there is clearly a marked anti- 

 pathy between the radio-elements and native iron, for 

 in all the terrestrial examples of the latter which have 

 been examined, uranium and thorium are barely 

 detectable. Correlating these facts with the earth's 

 trizonal structure, we should expect an internal 

 metallic core free, or nearly so, from the radio- 

 elements, an intermediate zone originally poor in, and 

 now almost devoid of, these elements, and an outer 

 crust more richlv endowed, its wealth of radium, like 

 its wealth of silica (and perhaps of manv of the 

 metallic ores), having accumulated in the course of 

 the earth's evolution at the expense of the more 

 sparsely distributed internal stores. 



On the parallel drawn by Daubr£e and extended by 



Suess between iron meteorites and the earth's metallic 

 core and between stony meteorites and the earth's 

 intermediate ultra-basic zone, this conception receives 

 still more convincing evidence. The stony meteorites 

 are even more poverty-stricken in radium than the 

 ultra-basic rocks, and the iron meteorites contain no 

 radium whatever. Arthur Holmes. 



Imperial College, London, S.W., July iS. 



Area of Earth's Surface Visible at any Altitude. 



In these days of aviators and of record heights 

 attained by them, perhaps the following rule to find 

 the area of the earth's surface visible from a given 

 height may be of interest. The rule depends upon 



the fact that if the height above a sphere is ' th part 



of the sphere's diameter, then the area visible from 



this height is — — th part of the sphere's total area. 



This admits of an easy geometrical proof. 



Rule. -Express the height above the earth's sur- 

 face as a fraction of the earth's diameter; multiply 

 the numerator of this fraction by 2, and add the 

 result to the denominator, then the resulting fraction 

 gives the fraction of the earth's surface visible. 



Examples. 



Height above earth's ' ' Fraction of 



, asafract f earth s total 



earth's diameter surface visible 



24,000 miles y '± 



8,000 ,, I 



7° „ gfa sh, 



' mile suW gotta 



506-881 in ,.,'„„. „;. 



42-24 ft „' ' — I— 



•+ * 1» 10' + 2 



At the moon / ,,, , ;o 



(240,000 miles) ( ' tu 



Of course, the effects of refraction are neglected ; 

 otherwise the rule is strictly accurate. 



W. Moss. 

 Municipal Secondary School, Bolton. 



Submerged Valleys and Barrier Reefs. 



As I have never visited 'the Pacific Islands, I do 

 not attempt to bring their valleys under the same 

 category as those of the coast of the Red Sea and 

 East Africa. Darwin's theory having been so often 

 held to apply to all barrier reefs, it seemed to me 

 interesting to refer to cases to which that theory 

 does not apply, though superficially resembling that 

 cited by Prof. Davis (Nature, February 6 and June 

 26). 



I wished also to emphasise the resemblance between 

 fault and subaerial erosion valleys, and in spite of 

 Prof. Davis's assertion that they can be distinguished 

 readily I think we need definite assurance that those 

 he cites are without doubt of the latter kind. Given 

 that assurance, Dana's proof of Darwin's theory holds 

 true for that case, but not universally wherever bar- 

 riers (and atolls) are found. I myself took it for 

 granted that those of Pemba and British East Africa 

 were due to erosion by streams and tides alone, until 

 I compared the better preserved examples of this 

 almost rainless climate. Cyril Crossland. 



Dongonab, Red Sea, July 12. 



NO. 2284, VOL. 91] 



