406 Mr. A. S. Davis on a Theory of Nebula and Comets. 



vacuum without evaporation. If, then, we suppose for a moment 

 that solid matter exists without being enveloped by gas, it will 

 immediately begin to evaporate and form an atmosphere about 

 itself. This atmosphere will be very rare and very extensive, 

 as the central mass, being comparatively very small, will exert 

 but a feeble attraction on it. 



Or, again, if we suppose that meteoric matter unenveloped by 

 gas would not acquire an atmosphere by evaporation, it would 

 do so in another manner ; for it is certain that some of the 

 meteoric bands approach very near to the sun in their perihe- 

 lion. These would attract to themselves a part of the sun's at- 

 mosphere, which they would carry away with them on their de- 

 parture ; and portions of this they would, in their turn, part with 

 to every meteor which came within a sufficiently small distance 

 from them. 



Graham has found that meteoric matter which has fallen to 

 the earth gives evidence of having been exposed, when at a high 

 temperature, to hydrogen existing under a pressure of several 

 atmospheres. 



We conclude, then, that the sun is surrounded by an enve- 

 lope of gas, which is not a true solar atmosphere, but is the ag- 

 gregation of the atmospheres of numberless meteoric bodies re- 

 volving around it. 



Now M. Hoek has shown that comets are detached portions 

 of large masses of matter ; and it has been suggested that these 

 large masses may be nebulae. Admitting this, a comet, before 

 its entrance into the solar system, will consist of a solid or liquid 

 nucleus surrounded by a large mass of very rare invisible gas. 

 On its approach to the sun, the nucleus will make its way most 

 easily into the solar envelope, and the comet will enter with its 

 tail directed away from the sun. A chemical combination will 

 take place between the tail of the comet and some of the gaseous 

 elements of the solar envelope; and where this combination oc- 

 curs, the gases will become visible from the light evolved, and, 

 if the compound formed be in a solid or liquid state, from the 

 light also which it reflects from the sun — or if, as probably would 

 be the case, the matter be in a state of minute subdivision, from 

 light scattered by that kind of dispersion which Professor Tyn- 

 dall has lately shown is produced by finely divided matter. 



On passing through its perihelion, the comet loses a great 

 part of its tail, which soon cools down and becomes mingled 

 with the rest of the solar envelope. 



On leaving the sun, the tail begins to increase, from the ad- 

 dition to it of matter rendered gaseous by the heat of the sun. 

 Those parts of the gas where chemical action has taken place 

 being heated, and therefore rendered specifically lighter than 



