May 20, 1910] 



SCIENCE 



773 



approaching the sun, a part of its orbit where 

 the temperature given by the solar radiation 

 to the surfaces of the masses is sufficient to 

 boil off or regasify the condensed material; 

 then not only is the gas blown off into 

 Tacuous space around the nucleus of the 

 comet, but it is naturally blown off in the 

 direction towards the sun, from the heated 

 side of each mass, and at the same time that 

 the gas leaves the mass other fine particles 

 are lifted by the force of the escaping gas. 

 This is due to the fact that these fine or 

 dust-like particles are not held with any 

 strong gravitative tendency. Ultra-violet 

 radiation may also add its effect in causing 

 discharge of negative ions. The result of this 

 is that jets or flows of materials from the 

 nucleus tend into the vacuum towards the 

 sun from the warmed or radiation absorbing 

 surfaces of the comet's nuclear masses. As 

 soon as they leave the nucleus or the warmed 

 surfaces, they are again cold and mainly con- 

 densed. But, though exceedingly fine, they 

 are now absorbers, more or less solid, of the 

 sun's radiation, and are gradually thrust 

 backward by the pressure of the light and 

 radiation and are blown off in the opposite 

 direction by this pressure, so forming a 

 tail in the contrary direction from the 

 sun, or in a direction opposite to that 

 in which they were first ejected. There 

 being in matter all grades of volatility, as 

 the cometary body approaches the sun, ma- 

 terial-more and more refractory, so to speak, 

 would be evolved, until finally, if the ap- 

 proach is near enough to the sun, even ordi- 

 narily solid substances would be vaporized 

 from the nuclear masses and projected to 

 form a tail, as has just been described. Some 

 of this vaporized matter would immediately 

 condense on getting a little farther away, and 

 form solid particles in the tail. The comet of 

 January, 1910, showed sodium lines, showing 

 that the temperature of the nuclear masses 

 had probably reached the vaporization point 

 of sodium. The greatest extension of a 

 comet's tail usually comes just after the comet 

 passes perihelion, because the heating process 

 keeps on, as it were, a little past perihelion. 



just as the hottest part of our summer days 

 is two or three o'clock in the afternoon. Now 

 if the comet stays in proximity to the sun 

 long enough, it will have discharged nearly 

 all of its volatile material for a particular 

 temperature reached. But on leaving the 

 sun after the tail has shrunk (which is a very 

 natural thing for it to do when the body 

 passes through regions less heated by solar 

 rays), it may again be in the condition to 

 gather up the condensed and practically solid 

 gases and vapors in the space around it. And 

 if its period is a long one, such as 2,000 

 years, as in the case of Donati's comet, it 

 should not surprise us if there is sufficient 

 material to form a fair tail, which only lasts 

 a few weeks at the most. Then it must be 

 borne in mind, too, that an extremely small 

 amount of material diffused in space under 

 solar radiation will suffice to form a very 

 large tail, as every particle, even of extremely 

 small mass, becomes substantially a light 

 source. Take, for instance, the amount of 

 tobacco smoke that can cloud up a room when 

 the sun is shining in it, and it will be found 

 to be a very small quantity, but, if the room 

 be black as night and a hole be made in a 

 shutter through which a small beam of sun- 

 light enters and the minutest body of smoke 

 be diffused in the room, there will be a 

 " comet's tail " extending from the opening 

 across the room where the sunbeam passes 

 because it will be seen in blackness and that 

 is the condition of our seeing comets' tails; 

 in the darkness of night. Then we must re- 

 member how deep the space is which is occu- 

 pied as a visible thickness in a comet's tail, 

 say, 50,000 miles. We thus get an idea of 

 how free of particles space must be not to 

 shine with a luminosity equal to that of a 

 comet's tail when we look off into the dark 

 night irradiated by the intense solar beams. 

 Doubtless the simple view here given is com- 

 plicated by many other actions, electric, etc. 

 Comet's tails sometimes vary greatly and 

 rapidly. We need not be surprised at this 

 when boiling points are known to be critical; 

 when, in other words, a few degrees increase 

 in temperature may vaporize a substance 



