194 WILLIAMSON OiV COMETS. 



sun, because the illuminated atmosphere, or tail of the comet, is con- 

 stantly thrown in opposition to the sun ; but it has been questioned whe- 

 ther the comet's atmosphere can be repelled by the rays of light? 

 When I formerly ventured an opinion, that comets are habitable, I 

 availed myself of a theory advanced by Kepler, that the rays of the 

 sun's light have the power of repelling particles of air. To this theory 

 it has been objected, that it cannot be supported by experiments, the 

 test of philosophical reasoning. I have doubts concerning the weight of 

 this objection ; because the rays of the sun, collected by a burning glass, 

 are observed, in our gross medium, to move a light body delicately sus- 

 pended. But if no such experiments had been attempted with success, 

 there are certain conclusions that we are bound to admit from our gene- 

 ral knowledge of matter and motion.. Thus it must be admitted that 

 every body, having great velocity, and striking against a body at rest, 

 makes some impression on the same. But it is ascertained that the 

 rays of light, coming from the sun, travel more than one hundred and 

 seventy thousand miles in a second of time. Let us now figure to our- 

 selves, a particle of matter, ever so small, and let us multiply its weight 

 into the velocity just mentioned, and this will produce a force to which 

 a particle of air must yield, in a free, celestial space. Now, as the par- 

 ticles of light are generally, as I believe, allowed to be some kind of 

 matter, and as the impetus of all matter, in motion, is according to its 

 velocity, we can easily perceive that the force of the rays of the sun 

 may be sufficient to repel the atmosphere of a comet. According to 

 this theory, the nearer a comet approaches to the sun, the more com- 

 pletely will it be stripped of its atmosphere, except that the attraction 

 of the comet, which is powerful near the nucleus, will necessarily detain 

 some part of its atmosphere. Such, too, is the fact; for the luminous tail 

 increases in length until, in some cases, it extends to sixty or eighty mil- 

 lions of miles. Hence it is that the cometarians, when they approach the 



