190 / WILLIAMSON ON COMET9. 



amounting to a four thousandth part of his bulk, in the space of one 

 thousand years ; especially as it is in the constant receipt of light 

 from other luminaries. 



Unless arguments shall be adduced, much more conclusive than any 

 thing I have seen, to repel the idea of the sun's light having a certain 

 force, I shall adhere to the original opinion advanced by Kepler. But 

 there are two other powers, either of which, as I conceive, may enable 

 us to account for the repulsion of the cometic atmosphere. I refer 

 to the repulsion that subsists between the particles of air, and a similar 

 property of the electrical fluid. 



It is generally believed that the electrical fluid is contained as well 

 in the sun as in every other body within our system ; and it is known 

 that the particles of that fluid are highly repellant of one another. 

 The experiment made by two cork balls, suspended near one another, 

 by threads of silk, is familiar to every electrician. Attach short threads 

 of flax to those balls, and electrize the balls; the balls recede from 

 one another, and the threads, repelled by the electrical fluid that is 

 escaping from the balls, fly off at a tangent. After viewing this simple 

 experiment, it is impossible not to perceive, with how much ease the 

 atmosphere of a comet may be repelled by the electrical fluid that 

 proceeds from the sun. 



The other power to which I referred, by which the atmosphere of 

 a comet may be driven behind its nucleus, is the repelling power that 

 subsists between particles of air. In this case I take for granted, as 

 the fact seems to be confirmed by observation, that the sun has an 

 iitmosphere. We have all observed that the particles of the atmos- 

 phere may be forced nearer to one another, but they strongly resist 

 «uch compression. Hence it is that air, forced into the barrel of 

 a gun, is known to discharge a bullet with deadly effect. Such, too, 

 is the tendency of every particle of air to recede from every other 

 particle, that, according to the allegation of Sir Isaac Newton, a single 



