134 THE PRESSURE DUE TO RADIATION. 



by the influx of negatively charged nuclei from the sun, which serve 

 as condensation centers for the ascending vapors. The height above 

 the nucleus of the comet at which this condensation would occur would 

 thus, in some measure, be governed by the supply of negative paiti- 

 €les. These would be found in increasing numbers with diminishing 

 distance from the sun. This action may be responsible for the contrac- 

 tion of the head and envelope as comets approach the sun. 



The brilliancy of the envelope may l)e attributed in large part to 

 the fact that bodies of sufficient size to reflect solar ravs are first 

 formed out of the vapors of the head in this region. The negative 

 nuclei from the sun would here experience an obstruction and lose the 

 greater part of their motion b}^ friction. P^lectrical interchanges and 

 discharges would be more active, and the hvdrocarbon spectrum be 

 brighter in tlie envelope than in other parts of the head. 



If the brightness of the head and its envelope depend upon the 

 number of negatively charged nuclei which strike the comet, and if, 

 as Arrhenius maintains, the nuclei move out from the sun radiallj^and 

 in greatest numbers from regions of greater solar activity, comets 

 crossing the surface defined by solar radii drawn through the sun-spot 

 belts should show a marked increase in brightness, especially in maxi- 

 mum sun spot years. The writers are not aware that any such 

 influence has been looked for in the cases where sudden changes of 

 briglitness in comets have been observed. 



Comet tails. — The maximum ratio of radiation pressure to gravita- 

 tion, obtained theoretically h\ Schwarzschild for sunlight upon opaque 

 reflecting spheres of 0.8 density, under the most favorable conditions, 

 was about 20 to 1, if the recent estimates (ranging from 3.5 to 4) of 

 the solar constant were used. 



In Bredichin's three types of cometary tails the highest ratio of 

 attraction to repulsion required is about 18 to 1. The multiple tails 

 observed in such comets as Donati's ma}^ thus be satisfactorily 

 explained by the sifting action of radiation pressure in two ways — 

 either by assuming, with Bredichin, that the particles in the different 

 tails are of different densities, but of uniform size, or hy assuming 

 uniform density and particles of several different sizes. 



While radiation pressure alone ma}' thus afford a satisfactory expla- 

 nation of comets' tails, there is no reason to assume that it is the only 

 cause of the repulsive action observed. There are several ways in 

 which the gases and vapors present in the tail may exert a force upon 

 the small solid or liquid particles which are known to exist there: 



1. Small particles, if warmed on one side when surrounded by gases 

 or vapors, even under pressures so low that electrical discharges take 

 place only under relativeh" high voltages, experience a strong repul- 

 sion, similar to that upon a vane of a Crookes radiometer. 



