On Comets' Tails. 345 



In the case of Hydrogen — 



A=8-8xl0-« 



.-. v=2 X 10~" nearly, 



so that this is still only one tenthousandth part of the mean dis- 

 tance between them, and is considerably less than the size of a 

 molecnle as usually estimated. Of course no gas would absorb 

 all the light that falls on it, but from these calculations it seems 

 possible that a light gas, with large molecules, such as some of the 

 complex hydrocarbons that absorbed a considerable proportion of 

 the radiations that fall on them might not only be in equilibrium 

 under the attraction of the sun and the repulsion of its radiations, 

 but might be repelled by the sun with considerable force. It has 

 been lately shown that a repulsive force emanating from the sun 

 would account for comets' tails. The force however must not be 

 like gravitation, proportional to the mass of the body, for some 

 comets have more than one tail, and this requires a different 

 accelleration on some kinds of the cometary matter from that on 

 others. This would be completely accounted for by the hypothesis 

 I have put forward. The repulsion of the sun depends on the 

 surface of the molecule and on its absorbing power for heat, and 

 these are not by any means proportional to its mass. 



Note added in Pkess. — 1st August, J 882. 



Another method can fee founded on calculating the proportion of heat 

 that should be absorbed by the unit volume of a substance in order that 

 it be neither attracted nor repelled by the sun. As before the pressure 

 on a completely absorbing unit volume would be — 



P=4-2 X 10-5, 



so that if a were the absorbing power the force on it would be — 



F=4-2 X 10-5 X a. 

 On the other hand the attractive force on a unit volume of density A 

 would be — 



F=-59xA. 



