18 Transactions Oh the 



medium, more or less heat would be developed, according to the velocity, 

 which would increase as the mass approaches the perihelion, while the resist- 

 ance and heat would increase still more rapidly, attaining a maximum soon 

 after passing that point. The heat thus developed and probably increased 

 by the proximity of the sun, would tend to volatilize some of the elements 

 of the body, causing them to become luminous and present the phenomena 

 of the corna and the nucleus. It is possible, too, that some of the elements 

 of the medium through which the body is moving would share in and 

 increase the luminosity. 



Now, with regard to the tail, it has been heretofore assumed that the train 

 so designated, often accompanying a comet, is an essential part of it. Bessel 

 thought, as we have seen, that the material elements of the tail were derived 

 from the nucleus flowing from it, and then repelled by a force centered in 

 the sun. Schiapparrelli and Le Verrier think these elements are of the same 

 kind of substance as that of the nucleus, but in a more diffuse condition, and 

 becoming luminous by some motion among themselves. 



But may it not be possible that this train of light Avhich accompanies a 

 comet, and has been called a tail, is, in fact, no part of the comet more than 

 a shadow is a part of the object which gives it form? May it not consist of 

 particles of the medium through which the comet passes, made luminous by 

 the rays of the sunlight acted on by the glowing gas of the corna ? If the 

 zodiacal belt forms this medium, in wdiole or in part, being already slightly 

 luminous, its luminosity (it is possible to imagine) might be easily increased. 



How the phenomenon of light is actually produced, we do not know. We 

 know that the flames of certain gases give rise to it — that is, cause the 

 necessary undulations of the ether, and we know that our sun is the source 

 of a large supply of light, and, though we may know some of the attend- 

 ant circumstances in these cases, yet who can tell the circumstances that give 

 rise to auroral light, or who can tell the origin of zodiacal light? 



So we may see the phenomenon of light in the train that accompanies the 

 comet, and, though we may not be sure of all the conditions that take place, 

 yet it seems highly improbable that this luminous mist forms any part of 

 the comet, and I predict that Avhen a brilliant comet gives an opportunity 

 to apply the tests of spectrum analysis, it will appear that this bright train 

 is not nebulous, as Bessel thought, and is not diffuse meteoric matter, as 

 Schiapparelli and Le Verrier have supposed, but is the illumination of the 

 medium through which the comet is passing. 



In closing, let us notice several facts that have interest in this connection: 



It was thought highly probable that the earth passed through a portion of 

 the tail of the bright comet of 1861, on the 30th of June, of that year. The 

 efifect was apparently to dim slightly the light of the sun, before sunset, and 

 at the same time to give the sky an " auroral, glare-like look." The comet 

 itself, in the evening, had a more hazy appearance than at any other time 

 after that evening. These incidents seem inconsistent with the theory of a 

 meteoric composition of the tail, but more consistent with the hypothesis of a 



