THE PRESSURE DUE TO RADIATIOIST. 133 



in the interior of the nucleus and the consequent shooting out under 

 pressure of a mixture of gases and dust through holes in a loose outer 

 crust. Lack of sufficient means of escape in this way may cause a 

 bursting of the nucleus sometimes observed. 



The general upward current of vapors from the nucleus to the envel- 

 ope, aside from jets, may be due to convection away from the more 

 strongly heated center. '^ 



Because of the counter-pressure due to the radiation of the nucleus 

 itself, the rising of even small solid particles from the nucleus to the 

 envelope would not encounter as strong an unbalanced pressure from 

 the solar rays as particles in the tail. For, if all the heat received 

 from the sun were again radiated from the nucleus on the side toward 

 the sun, these two counter-radiation pressures would exactly balance 

 at the surface of the nucleus. * 



Small particles ma}^ also be aided in rising from the nucleus toward 

 the sun by gas forces. By numerous experiments on larger bodies 

 immersed in a gas and illuminated on one side, it has been shown that 

 they may be either repelled from the light source or drawn to it, 

 depending upon the pressure of the surrounding gas. (See curves, in 

 the foregoing paper, fig. 4.) If the gas pressure is not too low, par- 

 ticles after leaving the nucleus might first be drawn toward the sun 

 until a region of higher vacuum was reached in the ascent, and then 

 be repelled.'' 



The brilliant envelope of the head may be regarded as forming at 

 the height where condensation, caused by expansion and cooling, takes 

 place. Here the repelling action of the solar radiation Avould reach a 

 high value and the particles in the envelope would be driven backward 

 to form the tail. 



According to Arrhenius'' this condensation in the envelope is assisted 



« Matter in the form of gases and vapors is not subject to radiation pressure, as 

 solid and li<inid particles are, because of the minuteness of molecular dimensions. 

 Except in the spectrum regions of characteristic absori^tion, radiation can, theoreti- 

 cally, exert no pressure whatever upon a gas. Hence gases might rise from the 

 nucleus toward the sun practically unhindered by radiation pressure. 



''It is worth noting in this connection that the longer and invisible waves are as 

 effective in producing pressure as the visible radiations, and that these long waves 

 strongly preponderate in the spectra of solid bodies at temperatures low in com- 

 parison with the solar temperature. 



c It is possible also that electrostatic forces may play a small part in the forma- 

 tion of the head from the nucleus. Arrhenius believes the sun to have a positive 

 electrical charge, due to the fact that it loses more negative electrons by condensation 

 into nuclei and subsequent rei)ulsion by radiation pressure than it does of positive 

 electrons which do not as readily serve as centers of condensation. Streams of nega- 

 tively charged particles would communicate a negative charge to the matter surround- 

 ing the comet's nucleus, which would thus be attracted by the sun. As this attrac- 

 tion would oi)pose the formation of the tail in the same measure as it assisted that of 

 the head, it can not be a dominating inHueiice. 



rfL.c, p. 208. 



