26 KANSAS CITY REVIEW OF SCIENCE. 



surrounding it is a comparatively dark substance, and next this last is a bright 

 envelope which covers the head of the comet on the side next the sun, and trails 

 off into space forming that remarkable appendage which renders these bodies so 

 conspicuous. This train or tail is in some instances projected to the most enor- 

 mous distaiices — that of the comet of 1680 attaining the length of 123,000,000 

 miles — a distance much exceeding that from the earth to the sun. 



The process of forming the nebulous envelopes and the tail often results in 

 giving to comets the most gigantic dimensions — the volume of the great comet of 

 1843, for instance, including, of course, its tail, being at one time more than three 

 times that of the sun. This immense size indicates either a vast amount or an 

 extreme tenuity of cometic matter. That these bodies contain but a small quan- 

 tity of matter was shown in the case of Lexell's comet of 1770. Lexell had cal- 

 culated the period of this comet to be about five and one-half years, yet the 

 comet has never since been seen. 



In seeking for the cause of its failure to appear as expected, it was found 

 that on the occasion of its first return in 1776 it was completely hidden by the 

 rays of the sun, and that in 1779 it approached so near the giant planet Jupiter 

 as to become, as it were, entangled among his moons, the effect of which ren- 

 contre was to deflect the comet entirely out of its orbit, so that it either no longer 

 moves around the sun in an ellipse, or if so, does not approach sufficiently near 

 to become visible to us. While the comet was so remarkably affected there was 

 not the slightest visible change produced in the motions of any of Jupiter's satel- 

 lites, which must have resulted had the mass of the comet been at all comparable 

 with that of those small bodies. 



We have seen that the volume of the great comet of 1843 was more than 

 three times that of the sun. 



Now the earth is about 5,000 times as dense as ordinary air and the sun 

 about one-quarter the density of the earth but contains about 314,000 times as 

 much matter — so that if the great comet of 1843 had been of the mean density of 

 ordinary air it would have contained nearly 1,000 times as much matter as the 

 earth. Now if this comet was at all analogous to that of Lexell, and there is 

 every reason to believe so, it could not have contained the yTyVo" P^^* ^^ ^^ mass 

 of the earth, so that its mean density probably did not exceed the y (jTy-^-oo-D- of the 

 density of the air we breathe. 



Comets being thus known to be bodies of small mass and great size it is 

 natural to suspect them to consist principally if not entirely of gas. There is 

 some question, however, as to whether a body composed entirely of gas could 

 exist m outer space : some holding that the repulsive properties of such a state 

 of matter would dissipate it indefinitely, to be disrupted and appropriated by bodies 

 more dense and rigid. Against this it may be said that " Marriotte's Law of the 

 Compression of Gases " indicates that the expansive force of a gas diminishes as 

 the cube of the distance between its particles increases, while the gravitation of 

 these particles toward each other is known to diminish as the square of their dis- 

 ances from each other increases. So that the expansion of a gaseous body in 



