November ii, 1920] 



NATURE 



54r 



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 cotnmunications.\ 



Restoration of Energy. 



Concerning the pressure of light, we may safely 

 say that it was predicted by Maxwell, discovered by 

 Lebedew of Moscow, and independently by Nichols 

 and Hull in America ; while it was elaborately dis- 

 cussed, clinched by further experiments, and its signi- 

 ficance greatly extended by Poynting and by Poynting 

 and Barlow. ' We now learn from Prof. Eddington's 

 brilliant address to Section A of the British Associa- 

 tion at Cardiff this year that radiation pressure has a 

 cosmic significance beyond what had been thought 

 possible ; that it holds back or sustains the outer sub- 

 stance of the brighter and hotter stars, and is respon- 

 sible for their huge size ; while at the same time it 

 has the effect of limiting the possible aggregation of 

 masses of matter, so that the mass reasonably per- 

 missible to any star ranges from five times to one- 

 half that of our sun — something of that order. 



.According to Prof. Eddington's calculations, the 

 radiation from an exceedingly hot central nucleus, 

 heated it may be by atomic disintegration, acts like a 

 rushing mighty wind on the outer portions, and 

 sustains them against gravitative attraction ; — surely 

 a remarkable example of the significance inevitably 

 attaching to the most minute and barely detectable 

 forces. So long as any given kind of force exists at 

 all, it may under proper conditions have an over- 

 powering and surprising effect. 



I write to convey a verbal suggestion made by my 

 assistant, Mr. Edward E. Robinson, a year or two 

 ago, that light-pressure may afford an escape from 

 some popular esrhatological conclusions based on the 

 doctrine of the dissipation of energy. Engineers Tke 

 Prof. Osborne Revnolds and .Sir William Siemens 

 have deplored the waste of solar energy, of which so 

 minute a fraction is caught by the earth or any planet, 

 and have sought to circumvent and reconcentrate it 

 somehow — by total reflection at an ethereal boundary 

 or otherwise. .And when we think of the vast store 

 of power ceasetesslv being radiated from every star, 

 and apparently fruitlesslv lost in the depths of space, 

 it is natural to look either for some usefulness in 

 the torrent or for some sort of compensating 

 mechanism. So also, under the influences of gravita- 

 tive attraction and the general law of dissipation of 

 energy, it has seemed to some as if the cosmos was 

 tending towards a cosmic tomb — consisting of one 

 large coM or cooling lump of matter, with all the 

 energv of its gravitative falling together wasted by 

 radiation into the depths of space, and no recovery 

 possible, nor anv more generation of heat. 



But what about the sweeping or propelling power 

 of that apparentlv waste radiation?' It exerts pressure 

 on matter; particles of suitable small size must be 

 swept along with it. Whv should we not contemplate 

 a constant sweeping of cosmic dust away towards 

 infinitv, with full power of return when sufficiently 

 re-aggregated? Is there not some hope of restitution 

 and restoration in tH^. so that gravitative fail can 

 onrp more rerur and the whole cvclc begin .again ? 



To estimate the amount of matter which can thus 

 br repelled, taking into arrnunt diffraction effects, is 

 rather comTTlirafe<f. It must not be enough at anv 

 one time to cause effective onar ity ; but, considering 



\o. 2663, VOL. 106] 



the speed generated by any small acceleration in free 

 space and the length of time available, the total 

 amount of repelled matter need not be inferior to the 

 amount of disintegrated material which collisions and 

 friction and eruptions and electrical repulsion are 

 likely to provide. Something preserves the trans- 

 parency of space ; may it not be this constant sweep- 

 ing away of dust? Far away from any source of 

 radiation' the particles would "not be hot enough to 

 repel each other, so there would be nothing to prevent 

 their beginning to collect together, and so preparing 

 to fall once more from practical infinity. 



Electrified particles, ions and electrons, of which 

 interplanetarv space must contain myriads, are also 

 propelled By' light. But these seem to attain a ter- 

 minal velocity, of value depending on the intensity 

 of the radiation and the square of its wave-length. 

 Long waves travelling in space would therefore be 

 most effective, but short waves would act in the right 

 direction ; and the electrons thereby driven among the 

 cosmic dust— exerting mutual forces much stronger 

 than gravitation— might act as the cement to weld it 

 together again. 



So. long as matter is being accelerated by radiation- 

 pressure, and so long as fresh ions are being produced 

 in its path, the energy of the radiation would tend 

 to be consumed. Hence the ultimate result of all the 

 otherwise waste radiation might be just that energy 

 of gravitative separation which is required for a new 

 Lu(Tetian universe. 



There are other possibilities, of too speculative a 

 character, depending on the semi-material nature of 

 light. The suggestions so far m;ule mav be nega- 

 tived, but thev seem worth putting forward in a tenta- 

 tive manner. Oliver Looge. 



British Laboratory and Scientific Glassware. 



I HAVE no interest in the manufacture of scientific 

 glassware, except in so far that as 1 devoted most of 

 my time to the subject during the war I should like 

 to' know that my work would lead to permanent 

 results. I may, therefore, be permitted to address a 

 word to users and manufacturers of scientific glass- 

 ware. 



To users 1 say : Use only scientific hollow-ware 

 which bears the maker's name; and if you find it 

 faulty, send it straight back to the manufacturer, 

 whom you will be assisting, and who will replace it 

 at once. Even the famous Jena glass was often 

 faulty, and while working in the Laboratory in the 

 autumn of 1914 it was not only once that Jena beakers 

 were found to have cracked without apparent cause. 

 On two occasions I actually heard beakers crack 

 while they were standing on the table. 



In the autumn of 1014 I worked out the resistance 

 glass and lamp-working glass which have be«'n .spoken 

 of as standing at the head of the list, and early in 

 lOiS I began to manufacture these glasses, and con- 

 tinued to do so throughout the war, maintaining the 

 original compositions, but modifying the batch 

 formulae As different materials became available. The 

 glasses were worked out on the basis of analyses of 

 a large number of foreign glasses and a study of their 

 properties. The investigation was a perfectly straight- 

 forward one. and no particular difficulty attached to 

 it, and I will even admit that lurk came to the aid of 

 judgment in arriving at the final conclusions. 



I am making this stateniint bor.-ujse I wish it to be 

 recognised that a misunderstanding exi.<)ts as to the 

 difTicuIties which had to be overcome in connection 

 with th* manufacture of scientific glas.sware, and not 

 because I wish to lav claim to particular credit for 

 carrving through a simple piece of work. The rofti 



