102 THE ROYAL SOCIETY OF CANADA 



extremely slow process of equipartition of energy with that of mole- 

 cules escaping from planetary and stellar atmospheres. 



As the above estimate of molecular density gives a total amount 

 of matter of the order 1 /38X earth's mass in a sphere having a radius 

 equal to that of Neptune's orbit it is improbable that the residual 

 gas we are considering could have a noticeable effect on planetary 

 motions. It might, however, be identified with the slightly resisting 

 medium whose existence has been thought necessary by some astron- 

 omers to account for the secular acceleration of Encke's comet,'^ 

 and which is considered by See^^ to have played an important role in 

 planetary and stellar evolution. 



The molecular density estimated above is very much less than 

 that conjectured to exist in some of the nebulae, 10^ molecules per cm.^ 

 being about the order of magnitude in this case." While the degree 

 of rarefaction which we have derived is very much greater than it is 

 possible to produce by any known physical means, ^* the total amount 

 of matter contained in regions of space of astronomical dimensions is 

 formidable: thus we find for the number of molecules in a cubic 

 parsec the estimate N = 3-75X10^'' Hydrogen molecules per parsec^. 

 Taking the density of Hydrogen at standard temperature and pressure 

 to be 0-0899 grammes per litre (containing 2-78X1022 molecules), 

 we obtain for the density of matter in interstellar space the estimate 

 1-21X10^^ grammes per parsec': as the sun's mass is approximately 

 1-96X10'' grammes we have finally for the density of interstellar 

 residual gas the estimate 6-3X10' sun's mass per parsec'. According 

 to Eddington^^ a reasonable estimate of the density of visible stars 

 in the neighbourhood of the solar system is 10 X sun's mass in a sphere 

 of 5 parsecs radius (525 parsec'): i.e., 0-019Xsun's mass per parsec'. 

 It follows that the density of "uncondensed" or "residual" matter 

 existing in interstellar space is of the order 10^ that of "condensed" 

 stellar matter. Even if, as there is some reason to believe, the num- 

 ber of "dark" stars is very much greater than the number of bright 



*^ On the recent history of this comet see a paper by Backland, "Encke's Comet, 

 1895-1908," Monthly Notices, 70 (1910), pp. 429-442. 



^^ See, T. J. J., "Researches on the Evolution of the Stellar System," 191U, 

 Vol. II.; pp. 134-158. 



" Henkel, F. W.; in an article "Nébuleuses et Essaims," "Scientia," Vol. 15, 

 (1914), pp. 294-307. 



1* The total number of molecoles per cm.^ corresponding to the vapour-pressure 

 of mercury at the temperature of liquid air is estimated at 3X10' (Dunoyer, M. L., 

 "Les Gaz ultra-raréfiés," in the collection "Les Idées Modernes sur la Constitution 

 de la Matière," Gauthier- Villars, Paris, 1913, p. 216). 



18 Eddington, A. S., "Stellar Movements and the Structure of the Universe," 

 Macmillan and Co., 1914, p. 255. 



