Arrhénius suggests a method by which microörganisms might be conveyed through intersidereal space with far greater dispatch and without any mineral vehicle such as a meteorite. He notes that particles of cosmic dust leave the sun as a coronal atmosphere and are propelled through intervening space by the pressure of radiation until they reach the higher atmosphere of the earth (viz. at a height of 100 kilometers from the surface of the latter), where they become the electrically charged dust particles of polar auroras (_v.g._ the aurora borealis). The motor force, in this case, is the same as that which moves the vanes of a Crookes’ radiometer. Lebedeff has verified Clerk-Maxwell’s conceptions of this force and has demonstrated its reality by experiments. It is calculated that in the immediate vicinity of a luminous surface like that of the sun the pressure exerted by radiation upon an exposed surface would be nearly two milligrams per square centimeter. On a nontransparent particle having a diameter of 1.5 microns, the pressure of radiation would just counterbalance the force of universal gravitation, while on particles whose diameter was 0.16 of a micron, the pressure of radiation would be ten times as great as the pull of gravitation. Now bacterial spores having a diameter of O.3 to O.2 of a micron are known to bacteriologists, and the ultramicroscope reveals the presence of germs not more than O.1 of a micron in size.[11] Hence it is conceivable that germs of such dimensions might be wafted to limits of our atmosphere, and might then be transported by the pressure of radiation to distant planets or stellar systems, provided, of course, they could escape the germicidal action of oxidation, desiccation, ultra-violet rays, etc. Arrhénius calculates that their journey from the earth to Mars would, under such circumstances, occupy a period of only 20 days. Within 80 days they could reach Jupiter, and they might arrive at Neptune on the confines of our solar system after an interval of 3 weeks. The transit to the constellation of the Centaur, which contains the solar system nearest to our own (the one, namely, whose central sun is the star Alpha), would require 9,000 years.