58 Prof. W. A, Norton on the Physical Constitution of the Sun. 



proaches the sun*. Now, if such an energetic force of repul- 

 sion emanates from the sun and operates on cometic matter at 

 all distances, both small and great, according to the law of in- 

 verse squares, there is assuredly a high probability that it may 

 play an important part on that vast arena where solar forces 

 are obviously engaged in fierce contention. It may be con- 

 jectured that the solar vapours are entirely different substances 

 from, and wholly unlike in their physical state, the cometary 

 vapours that appear to be so exceedingly subtle. But it is 

 certainly more philosophical to suppose that the same sub- 

 stances, or substances possessed of the same general properties, 

 are present in all cosmical bodies and the earth. Besides, we 

 are not without direct evidence on this point. Huggins, by ex- 

 amining the spectrum furnished by the light emitted from the 

 Comet II., 1868, detected the presence of the vapour of carbon 

 in the brighter portions of the comet. " He has been able to 

 discriminate between the light of the nucleus of a comet and 

 that of its tail. The nucleus is self-luminous, and its substance 

 is in the form of ignited gas. The coma shines by reflected 

 light as clouds do." 



If, as is now conceded by astronomers, the tail of a comet 

 is made up of matter detached from the general mass of the 

 comet by reason of a repulsive action exerted by the sun, it 

 must also be admitted that the matter expelled is not all urged 

 away by the same intensity of force and with the same velocity ; 

 for we find that it is much more widely dispersed in the plane 

 of the cometary orbit than is consistent with this supposition. 

 For example, I have shown in my theoretical discussion of Do- 

 nates comet f that, if we conceive particles of matter to have been 

 expelled from this comet with a certain small lateral velocity, 

 and urged away during a certain interval of time by a solar re- 

 pulsion bearing to the force of gravitation the ratio of 1'213 to 

 1, they would at the end of the interval have been found distri- 

 buted over a narrow band coincident at its forward line with 

 the curved preceding side of the tail of the comet, and that the 

 other portions of the tail must have been composed of matter 

 subject to various degrees of solar repulsion less than this. In 

 fact, the definite conclusion was in this way reached that the 

 preceding half of the tail consisted of matter repelled from the 

 sun with a force varying between the limits 1*213 and 0, and 



* This is generally, if not universally, admitted by astronomers. The 

 author has undertaken in former Numbers of Silliman's Journal to establish 

 by rigorous calculation that the luminous train of Donati's comet was deve- 

 loped by a force of solar repulsion cooperating with the attraction of gra- 

 vitation, both varying according to the law of the inverse squares. (See 

 Silliman's Journal, vol. xxxii.) 



f Silliman's Journal, II. vol. xxxii. pp. 54-66. 



