W. A. Norton — Physical Constitution of the Sun. 399 



1-213 to 1, they would, at the end of the interval, have been 

 found distributed 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 that the following half was for the most part 

 composed of matter detached from the comet simply by reason 

 of a weakened gravitation toward the sun,— the intensity o 

 the force of gravitation along the following side of the tail 

 being 0-455. There would seem then to be no alternative but 

 to admit that the tail of Donati's comet was composed of different 

 substances, (or else of one substance in different physical states) 

 subject to a repulsive action from the sun of various degrees ot 

 intensity; and either prevailing over the sun's attraction ol 

 gravitation, or partiallv counteracting it, and so giving rise to 

 an effective repulsion "'for certain of these substances, and to 

 a diminished gravitation for others. The simplest theoretical 

 explanation that can be given of this state of things is to sup- 

 pose that the solar repulsion consists of a series of impulses propa- 

 gakd in waves through the ethei- of space, and taking effect upon 

 atoms of different sizes with varying intensity. It is obvious tnat 

 if this be true, the smaller the atom the more effective should 

 be the repulsion as compared with the gravitating force solicit- 

 ing the atom ; since the ratio of the two forces should be pro- 

 portional to the surface divided by the volume of the atom -- 

 assuming that the mass is proportional to the volume or tnat 

 all atoms have the same density. We are thus incidentally led 

 to infer that the larger comets consist of a variety of substances, 

 hkethe earth. ^ . n 



The question now arises what can be the origin of the force 

 of solar repulsion. There is another side of the diversihed 

 picture presented by cometary transformations under tlie su .^ 

 influence, which gives some intimations on this P^^^^ JJ«^ 

 only is a certain pfrtion of the cometary matter repelled by the 

 snn, but it is also repelled by the nucleus of the comet 

 see in large comets, a series of envelopes rise at '^^^^^'^^^^ 

 tie nucleus, on the side turned toward the sun f f^JX ^^f/^ 

 nearly uniform rate until they become dissipated by t^^ sun s 

 repulsion. Luminous jets also stream out at times from the 

 ^e side of the nucleus. These phenomena, it cm n^^"" ^^, 

 ^loubted, are in some way the effect of the suns heat, ihe 

 simplest'and most probabfe conclusion is that the ejecting ^^^^^^^^ 

 ^tich is brought into play by the sun's heat is tlie direct repul 

 sive energy of the heat received by the comet We are thus 



