750 KANSAS CITY REVIEW OF SCIENCE. 



To account for the backward curvature of the jets and the peculiar form and 

 direction of the tail, we must look for some additional force. In all probability 

 this force resides in the sun, and is directly opposite in its effects to the power of 

 gravitation. But since the body of the comet obeys the law of gravitation with 

 sufficient fidelity, we must find a repulsion which sensibly acts only on the mole- 

 cules of gas or vapor. 



The only force suggested by experience as competent to these requirements 

 is that of electrical repulsion. Any one can prove for himself that two bodies 

 similarly electrified mutually repel each other. We know that the earth, through 

 effects of constant evaporation and other causes, is to some extent an electrified 

 body. For the same reasons, we should expect comets to be electrified in a 

 much higher degree. The sun itself certainly exerts an influence upon terrestrial 

 magnetism. Violent commotions on his surface have occurred at the same time 

 with unusual disturbances of the magnetic needle. Electrical repulsion acts in 

 proportion to surfaces and not to volumes. On particles of matter in a state of 

 infinitesimal subdivision it might act most powerfully, while not affecting a large 

 body to an appreciable degree. 



THEORY OF FORMATION OF TAILS. 



If, then, we suppose the sun and comets to be sufficiently and similarly elec- 

 trified, we have the force necessary to produce the backward curvature of the 

 jets, and to drive off the smallest and probably outermost molecules of the coma 

 to form the tail. Since, according to our hypothesis, very little matter can be 

 given off from the shaded side of the nucleus, we readily perceive why the tail 

 should be hollow in appearance. 



The orbit of the moving nucleus being curved, it is evident that the particles 

 driven off at any time with less than infinite velocity, would continually fall more 

 and more behind the prolongation of a line through the sun and comet — just as 

 has been observed. If the matter contains molecules varying considerably in 

 size, the larger ones would be driven off with less velocity. These would curve 

 backward more than would the lighter molecules driven off at the same time ; and 

 so we have the multiple tails which have been seen, as well as the classification 

 already described. Elaborate examinations of their average observed direction 

 and form suggest that each class may be composed of chemical elements peculiar 

 to itself. We may even venture to suppose that the tail of greatest velocity and 

 least inclination is composed of hydrogen. The second type may contain carbon, 

 with or without other elements; and among those of the third, chlorine would 

 most likely be found. 



It is a common error to suppose that this hypothesis, as to the formation of 

 the tail, requires a repulsive force of inconceivable power. The straightest tails 

 which have been observed are accounted for by supposing a repulsive force not 

 much greater than twelve times the sun's attractive power. The tails most fre- 

 quently seen (scimeter-like in form) may be produced by a force about one-ninth 



