136 



THE PRESSUEE DUE TO RADIATION. 



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as a pile of apples from the same tree. These spores were light, cel- 

 lular structures, tilled mainly with oil. They were calcined by heat- 

 ing to redness and all the vaporizable material driven off, leaving only 

 sponge-like charcoal spheres behind. The densit}^ of a mass of these 

 spheres (individuals could obviously not be dealt with) was measured 

 and found to be about one-tenth that of water. Making liberal allow- 

 ances for the spaces between spheres in the pile, the density of a sin- 

 gle sphere could not exceed 0.15." 



These spores, together with a quantity of emery sand, were placed 

 in a glass tube the form of which was suggested b}^ the hourglass. 

 A Smaller tubes led off from either 



end. One of these was fused to a 

 good mercury pump of the Geissler 

 t3'pe, the other ])ent down and joined 

 to a small flask containing mercury. 

 All of the tubes were wrapped with 

 wire giiuze and heated to a tempera- 

 ture just below the softening point 

 of glass, and the pump was worked 

 man}' hours. W hen the pump showed 

 no further signs of gas the mercury 

 in the flask was boiled and mercury 

 vapor driven through the tubes to 

 carry off any permanent gases which 

 the pump alone could not reach. 

 After this had continued for an hour 

 or more the tube system was sealed 

 ofl' from the pump and the mercury 

 flask was surrounded b}' solid carbon 

 dioxide and ether, and the hourglass 

 still heated. In this wa}' all of the 

 mercury vapor which could be con- 

 densed at a temperature of — 80° C. 

 was drawn out of the tubes. After 

 ^i«i- nearly an hour the mercury flask 



with its frozen contents was sealed off from the hourglass. 



The hourglass was then held in a vertical position and a beam of 

 light of approximately known intensity was directed horizontallv on 

 the lower half of the tube just below the neck, fig. 1. By tapping the 

 tube a fine stream of sand and charcoal puffball spores descended. 

 The sand particles fell through the beam, showing no deflection, but 

 the sj^ores were driven from the stream sidewise in passing th(^ beam. 



^According to Schwarzschild's formula, the ratio of radiation pressure to solar 

 gravitation for spheres of the size and density of these spores would he about 6 to 1. 



