LIGHT 



58 ETERNITY OF LIFE 



light exerts pressure on those objects on which it falls and, 

 furthermore, he determined the magnitude of this pressure. 

 >N^ It turned out to be infinitesi- 



/ ^ mal. The sunlight falling on 



' * the surface of the Earth only 



exerts a pressure equivalent to 

 0-5 mg/m^, but even this is 

 enough to cause minute par- 

 ticles of dust to move through 

 a vacuum at a considerable 

 speed. 



Fig. 1 is a diagram illustrat- 

 ing the experiment of Nichols 

 and Hull which demonstrates 

 UGHT f]^e theory well. They used a 

 LIGHT glass vessel shaped like an hour 

 glass. In it they placed a 

 mixture of emery and very fine 

 carbon dust obtained by the 

 carbonisation of fungal spores. 

 The air was evacuated from the 

 vessel. The stream of particles 

 falling through the narrow 

 opening was illuminated by a powerful source of light. The 

 emery fell to the bottom but the carbon particles were 

 diverted on to the walls. 



Arrhenius drew a picture of the passage of small particles, 

 among them the spores of micro-organisms, through inter- 

 planetary and interstellar space. Upward currents of air, 

 which would be specially strong after volcanic eruptions, 

 might carry particles of matter to very great heights, up to 

 100 or more kilometres above the surface of the Earth. In 

 the upper layers of the atmosphere there are, for a number 

 of reasons, constant electrical discharges which would be 

 more than enough to drive these particles of matter out of 

 the atmosphere of the Earth into interplanetary space. Here 

 the particles \sould travel further and further under the 

 one-sided pressure of the rays of the Sun. 



As from the surface of the Earth so, in the same way, very 

 small particles must be constantly becoming detached from 



Fig. 1. Diagram of the experi- 

 ment of Nichols and Hull. 



