544 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



in the course of the last few years. Perhaps it is fortunate that the 

 famous astronomer did not know these "new facts." They would 

 have destroyed the unity of the system of the universe as he saw it, 

 and the complication introduced by instances of exception would 

 doubtless have prevented hmi from formulating his hypothesis, so 

 grand in its unaffected simplicity. 



To-day we know that the Laplace theory must be modified m 

 some pomts. As a whole, however, it is still in force; it is a citadel 

 which in spite of everything resists all assaults, as II. Pomcare has so 

 well said. It is enough, therefore, to reconcile it with the new con- 

 quests of science; that is what the illustrious physicist of Stockholm, 

 Prof. Svante Arrhenius, has done. 



The Swedish scientist introduced into the theory of the evolution 

 of worlds a second force as necessary to consider as universal gravi- 

 tation, that is, the 'pressure of radiation, the conception of which is 

 due to J. Clerk-Maxwell, and the reality of which has been demon- 

 strated by the experiments of Lebedeff. This pressure is exerted 

 upon every surface exposed to a radiation by the very action of this 

 radiation; it is equivalent, in the immediate neighborhood of the 

 solar surface, to nearly 2 milligrams a square centimeter. 



As the dimensions of a very small spherule of matter decrease, the 

 importance of the surface in comparison with the mass increases at 

 the same time. Now the attraction of gravitation is dependent on 

 the mass, while the pressure of radiation is dependent on the extent 

 of surface. One can readily conceive, therefore, that in the case of 

 very tenuous particles the pressure of radiation may exceed the 

 attractive force of gravitation; in the case of nontransparent spherules 

 the 0.0015 of a millimeter in diameter the two forces are in equilib- 

 rium; and if the diameter of the particle falls below this amount, the 

 repelling force is the stronger and the particle is driven away from 

 the radiating body. On tiny particles whose diameter would amount 

 to as little as the 0.00016 of a millimeter, the pressure of radiation would 

 be ten times as great as the attracting force. These dimensions are 

 realized in the spores of bacteria. The small mass of these micro- 

 scopic granules increases the importance of their surface, and the 

 resistance of the air has such force over them that this tiny mass 

 dropped into the air would not fall a hundred meters in a year. The 

 slightest wind carries them off into the atmosphere and may take 

 them to the limits of our gaseous envelope, where the pressure of 

 the air is not more than a very small fraction of a millimeter of mer- 

 cury; that is, to an altitude of 100 kilometers. 



It is tins pressure of radiation that Arrhenius has given a place in 

 the formation of worlds. It drives away from the stars the fine 

 "cosmic dust" which the constant eruptions of these incandescent 

 stars throw out every moment; especially, it is this dust which con- 



