476 PROCEEDINGS OF THE ACADEMY OF [August, 



driven away from the nucleus by the pressure of light. In fact, I 

 have talked of this very thing with the astronomers, and they have 

 not raised any objections to it. I take it that what happens is about 

 this — at least, this is in accordance with my idea of what happens: 



When the comet is far removed from the sun and consists, as it 

 probably almost always does, of a nucleus of fairly heavy pieces 

 surrounded by lighter masses, they are, on account of the cold of 

 space, at a very low temperature, and what little gravitational effect 

 is produced is just sufficient to prevent them being scattered. They 

 are free, at the far distance from the sun, from anything like tidal 

 action. The cluster might even be revolved without being distorted 

 by disturbance of the parts. Should they pass within range of the 

 gravitational effect of Jupiter and have the path slightly disturbed 

 thereby, there would necessarily be (by the slipping of the parts over 

 each other) an attrition or grinding action taking place, and this 

 would continue so- long as the body was within the range of the 

 disturbing planet. The same would be true of the cometary clusters 

 passing towards the sun and around it (making a so-called perihelion 

 passage), except that there would be some little difference. In this 

 case, the particles of fine material, the result of the grinding, and held 

 in the cluster, would now be free. The manner of this freeing can 

 easily be understood. The face of the mass of the cluster on the side 

 toward the sun would undergo a warming or heating process. This 

 would result in the evolution into the vacuous space of whatever vapors 

 or gases, however rare they might be (originally occluded in the face 

 of the masses of the cluster), especially from the warmed surfaces. 

 This would amount to a flowing of gas or evolution of gas on the side 

 toward the sun which would lift the dust particles away from the 

 mass, and the very fine particles would thus be free to be driven back 

 by the light pressure to form a tail. It is an old idea that during this 

 process if the sun were highly electrified electrical actions would 

 take place, electrical readjustments to the increasing actions of the 

 sun, but I think it doubtful whether the tail is much, or anything, 

 of an electrical nature, for the reason that the vacuous space in which 

 the tail moves is so high a vacuum that no conduction would be 

 possible. 



I am inclined to think that whatever actions of temperature or 

 electrical actions occur would be practically within the nucleus or 

 near it. The spectroscope shows that as the comet approaches 

 the sun gas is actually evolved; and the spectra of cyanogen and 

 carbon monoxide are common in comets, and possibly also hydro- 



