§44 Scientific Froceedings, Royal Dublin Society. 



XXXII.— ON" COMETS' TAILS. By GEO. FEAS. FITZGEEALD, 



M.A., F.T.C.D., &C. 

 [Read May 5, 1882.] 



In the last edition of Clerk Maxwell's Electricity and Magnetism, 

 page 402, § 793, he calculates that, according to the electro- 

 magnetic theory of light, sunlight should exert on a body that 

 entirely absorbed it, a pressure equivalent to — 



P=:4'22 X 10~^ dyiies per sq. centim. 



Now at the distance of the earth the attraction of the sun for 

 a mass m is — 



y='59 X m dynes. 



These forces might balance if the absorbing surface were suffi- 

 ciently large in proportion to the mass, so that if s were the 

 surface — 



sx4-22 X 10-5=-59 xni 

 or 



m=s X 7 X 10~^. 



Now the mass of a molecule of a gas of density A at the ordinary 

 pressure and temperature is — 



m= A X 10~^^ 

 so that — 



5=10-17 X 1-4 X A 



This gives the surface of a molecule of a gas whose density is A 

 in order that it should be neither attracted nor repelled by the 

 sun. If this were the case at one distance it would be so at all 

 distances, because both the attraction of the sun and the intensity 

 of the light vary inversely as the square of the distance. To find 

 whether this is possible we must make some assumption as to its 

 shape, and I will assume that the molecule is spherical, which 

 will have as small a surface as possible, and apply it to the case 

 of Hydrogen. In this case assume v the radius of a molecule. 

 Then evidently — 



s=Trv^==3'l X v2^14 X 10-'^ X A 

 .-. 1/^=44 X 10^'« X A 



