439 



NA TURE 



[September: 3, 1903 



means should be taken to improve the attendance; and to 

 encourage homework. The polytechnics are to some extent 

 fed by students from these evening classes, and it is of 

 importance that their work should be as serious as possible. 

 Dr. Hoffert is able to report considerable progress in the 

 organisation of higher education in the eastern division of 

 England, especially the increased attention now being paid 

 fo the improvement of secondary education. In another 

 place Dr. Hoffert refers to the question of higher elementary 

 schools, and expresses the opinion that schools of this type 

 might verv profitablv be distributed at suitable intervals 

 over London. " They appear destined to fill an important 

 place in any future 'organised scheme of elementary and 

 secondary education, and to form the natural completion 

 of the elementary system." 



SOCIETIES AND ACADEMIES. 



London. 



Royal Society, June i8. — " Radiation in the Solar System : 

 its Effect on Temperature and its Pressure on Small 

 Bodies." Bv J. H. Poynting:, Sc.D., F.R.S., Professor 

 of Pfiysics iri the University of Birmingham. 



Part L — Temperature. 



We can calculate an upper limit to the. temperatures of 

 fully absorbing or " black " surfaces receiving their heat 

 from the sun, and on certain assumptions we can find the 

 temperatures of planetary surfaces, if we accept the fourth 

 power law of radiation, since we know approximately the 

 solar constant, that is, the rate of reception of heat from 

 the sun, and the radiation constant, that i§, the energy 

 radiated at i° abs. by a fully radiating surface.^ 



The effective temperature of space calculated from the 

 very uncertain data at our command is of the order io° abs. 

 Bodies in interplanetary space and at a much higher tempera- 

 ture may, therefore, be regarded as being practically in a 

 zero temperature enclosure except in so far as they receive 

 heat from the sun. 



The first case considered is that of an ideal earth, more 

 or less resembling the real earth, and it is shown that the 

 temperature of its surface is, on the average, 325°, 302°, or 

 290° abs. according as we take for the solar constant 

 Angstrom's value 4 cal./min., Langley's value 3 cal./min., 

 or a value deduced from Rosetti's work 2-5 cal./min. The 

 lowest value found, 290° abs., is very near the average 

 temperature of the earth's surface, which may be taken as 

 289° abs. As the earth's effective temperature must, if any- 

 thing, be below this, and cannot differ much from that of 

 the ideal planet, Rosetti's value for the solar constant, 

 2-5 cal./min. or 0175x10' ergs. /sec. is probably nearest 

 to the true value, and is therefore used in the following 

 calculations. 



The preceding calculations may be turned the other way. 

 It is shown that, on certain assumptions, the effective 

 temperature of the sun is 215 times that of the ideal earth. 

 If we consider that the real earth with a temperature 289° 

 abs. sufficiently resembles the ideal, we get a solar tempera- 

 ture 21-5x289=6200° abs. 



The upper limit to the temperature of the surface of the 

 moon is determined and is shown to be 412° abs. when no 

 heat is conducted inwards. But Langley finds that the 

 actual temperature is not much above the freezing point 

 on the average. This leads us to the conclusion that it is 

 not higher than four-fifths the highest possible value, the 

 reduction being due to inward conduction. 



The temperature of a small body, dimensions of the order 

 of I cm. or less, but still so large that it absorbs radiation, 

 is shown to be nearly uniform, and at the distance of the 

 earth from the sun about 300° abs. 



Under otherwise similar conditions temperatures -must 

 vary inversely as the square root of the distance from the 



1 W. Wien (" Cong. Int. de Physique," vol. ii. p. 30) has pointed out that 

 btelan s law enables us to calculate the temperatures of celestial bodies 

 which receive their light from the sun, by equating the energy which they 

 radiate to the energy which they receive from the sun, and remarks that 

 the temperature of Neptune should be below -200° C. 



sun. Thus Mars, if an earth-like planet, has a temperature 

 nowhere above 253° abs., and if a moon-like planet, the 

 upper limit to the temperature of the hottest part is about 

 270°. 



Part II. — Radiation Pressure. 



The ratio of radiation pressure due to sunlight to solar 

 gravitation increases, as is well known, as the receiving 

 body diminishes in size. But if the radiating body also 

 diminishes in size, this ratio increases. It is shown that if 

 two equal and fully radiating spheres of the temperature 

 and density of the sun are radiating to each other in a 

 zero enclosure, at a distance large compared with their 

 radii, then the radiation push balances the gravitation pull 

 when the radius of each is 335 metres. If the temperature 

 of two equal bodies is 300° abs. and their density i, the 

 radius for a balance between the two forces is 19-62 cm. 

 If the density is that of the earth, 55, the balance occurs 

 with a radius 3-4 cm. If the temperatures of the two are 

 different, the radiation pressures are different, and it is 

 possible to imagine two bodies, which will both tend to 

 move in the same direction, one chasing the other, under 

 the combined action of radiation and gravitation. 



The effect of Doppler's principle will be to limit the 

 velocity attained in such a chase. The Doppler effect on a 

 moving radiator is then examined, and an expression is 

 found for the increase in pressure on the front, and the 

 decrease in pressure on the back of a radiating sphere of 

 uniform temperature moving through a medium at rest. 

 It is proportional to the velocity at a given temperature. 

 The equation to the orbit of such a body moving round the 

 sun is found, and it is shown that meteoric dust within the 

 orbit of the earth will be swept into the sun in a time com- 

 parable with historical times, while bodies of the order of 

 I cm. radius will be drawn in in a time comparable with 

 geological periods. 



" The Phenomena of Luminosity and their possible 

 Correlation with Radio-Activity. " By Henry E. ' Arm- 

 str-ongr, F.R.S., and T. Martin Lowry, D.Sc. ' 



The possibility of regarding luminous manifestations 

 generally — including radio-activity — as the outcome of 

 oscillatory changes in molecular structure was pointed out 

 by one of the authors more than a year ago in a com- 

 munication to the Society in which the kind of change 

 contemplated was exemplified by reference to the case of 

 nitrocamphor. As the phenomena of radio-activity are 

 exciting so much interest, it is thought desirable to enter 

 somewhat more fully into an explanation of the argument 

 underlying this conception of the origin of luminous appear- 

 ances. 



In the note referred to, it was suggested that tribo- 

 Itiminescent substances, i.e. substances which become 

 luminous at the moment of crushing, might conceivably, at 

 the same time, manifest radio-activity. Sir William 

 Crookes, at Dr. Armstrong's request, has recently examined 

 saccharin from this point of view. 



His remarks are described; they seem to show that 

 saccharin is slightly radio-active towards barium platino- 

 cyanide when crushed. The authors have been unable 

 hitherto to detect any effect on the electrometer. 



Triboluminescence. — The authors consider the nature of 

 the change involved in the production of the luminous flash, 

 in order that it may be clear why, in their opinion, if radio- 

 activity were observed in such a case, it would have been 

 as the concomitant to chemical change. 



There is distinct evidence, they think, that the pheno- 

 mena of triboluminescence may be correlated with the 

 occurrence of the form of. isomeric change which attends 

 the passage of a compound into the isodynamic form of 

 lower potential. Tschugaeff, who has examined more than 

 500 inorganic and organic compounds, found that about 25 

 per cent, of the latter gave a more or less intense flash 

 when crushed ; of these a considerable proportion appear 

 to be such as could exist in isodynamic forms. Onlv about 

 5 per cent, of the inorganic substances flashed. 



To take the case of saccharin, the two conceivable forms 

 are : — 



CO\ /C.OH, 



NO. 1766, VOL. 68] 



Coiii\ 



^SO, 



>NH 



CfiHZ. 



\ 



-SO, 



