COMETS' TAILS. 267 



by the pressure of light at the sun's surface, is thus approximately fi. 

 For a spherical drop the critical diameter may be calculated to be 1.5 /i 

 for water. For other substances the critical value is inversely propor- 

 tional to the specific gravity. 



A similar effect of extreme minuteness is familiar to us as the ex- 

 planation of the long time required by very small particles to settle 

 through the atmosphere, amounting to many months in the case of the 

 finely divided dust thrown up during the eruptions of Krakatoa. But 

 the resistance to suspended dust particles can never exceed their weight, 

 since it is only called forth by the motion produced by the weight itself. 

 The pressure of light now considered may enormously exceed the weight 

 provided the particles are small enough. 



From the motions, and especially the curvature, of comets' tails the 

 magnitude of the repulsive forces to which they are subject may be cal- 

 culated. Thus Bredichin finds, in four instances, that the repulsion 

 must have been about 18.5, 3.2, 2.0, and 1.5 times the sun's gravita- 

 tional attraction. Now the vapors emitted by comets are largely 

 hydrocarbons of specific gravity about .8. To account for these repul- 

 sions on Arrhenius' principle, the drops must have had diameters of 

 0.1//, 0.59 fx, 0.94 (X, 1.25 fi respectively. In another case, where the 

 tail curved towards the sun, Bredichin found the repulsion to be 0.3 

 times gravity. This would indicate particles of diameter 6 //. Particles 

 of this order of magnitude, and far smaller, are familiar enough to us, 

 especially in combustion and in the early stages of condensation. 



The theory suggested is then as follows: As the comet approaches 

 the sun, the intense heat causes a violent eruption of hydrocarbon vapors 

 on the side towards the sun. The hydrogen boils off, and the vapors 

 condense into small drops of hydrocarbons with higher boiling-points, 

 or ultimately solid carbon is thrown out, finely divided as in an ordinary 

 flame. The largest of these particles fall back to the comet, or if they 

 are not condensed till at a great distance from it, they form tails turned 

 towards the sun. The smaller are driven rapidly from the sun by the 

 pressure of its light, with a speed depending on their size, and form the 

 ordinary tails pointing away from it. That particles of different sizes 

 should be formed from the same comet is natural since the comet is 

 likely to be formed of heterogeneous materials, and there must be great 

 variety in the circumstances of condensation. Thus the comet of 1744 

 had no less than five tails of different curvature. Occasionally the cal- 

 culated repulsion on the same tail is not found to follow exactly the law 

 of the inverse square of the distance from the sun throughout its whole 

 length. This puzzling circumstance is at once explained, if the particles 

 should for any reason change their state of aggregation, and conse- 

 quently their size, during their headlong career. In the light of this 

 theory the following passages will be found very suggestive. 



