Prof. W. A. Norton on the Physical Constitution of the Sun. 59 



that the following half was for the most part composed of matter 

 detached from the comet simply by reason of a weakened gravi- 

 tation toward the sun, the intensity of the force of gravitation 

 along the following side of the tail being 0455. There would 

 seem then to be no alternative but to admit that the tail of 

 Donates 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 of intensity, and either 

 prevailing over the sun's attraction of gravitation or partially 

 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 suppose that the sola)' repulsion con- 

 sists of a series of impulses propagated in waves through the cether 

 of space and taking effect upon atoms of different sizes ivith vary- 

 ing intensity. It is obvious that, if this be true, the smaller the 

 atom the more effective should be the repulsion as compared 

 with the gravitating force soliciting the atom, since the ratio of 

 the two forces should be proportional to the surface divided by 

 the volume of the atom, assuming that the mass is proportional 

 to the volume, or that all atoms have the same density. We 

 are thus incidentally led to infer that the larger comets consist 

 of a variety of substances like the earth. 



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

 of solar repulsion. There is another side of the diversified 

 picture presented by cometary transformations under the sun's 

 influence, which gives some intimations on this point. Not 

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

 sun, but it is also repelled by the nucleus of the comet. We 

 see in large comets a series of envelopes rise at intervals from 

 the nucleus on the side turned toward the sun, and recede at a 

 nearly uniform rate until they become dissipated by the sun's 

 repulsion. Luminous jets also stream out at times from the 

 same side of the nucleus. These phenomena, it can hardly be 

 doubted, are in some way the effect of the sun's heat. The 

 simplest and most probable conclusion is, that the ejecting force 

 which is brought into play by the sun's heat is the direct repul- 

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

 led to infer that the repulsive action exerted by the sun upon mat- 

 ter in the state of the cometary vapours probably consists, either 

 wholly or partially } in repulsive impulses propagated in the heat- 

 ivaves proceeding from the sun. 



Let us now see whether any confirmation of these inferences, 

 and any additional light in the direction of our present inquiry, 

 can be obtained on the substantial ground of terrestrial physics. 

 The definite question presented for consideration is whether the 



