GEOLOGY. 363 



eleventh magnitude, whilst the luminosity of the atmosphere illuminated 

 only by the moon is sufficient to render invisible, stars which are 250 times 

 more brilliant. 



The disproportion becomes still more striking when we consider, that ac- 

 cording to the measurement; of Wollaston, to which Sir John llerschel says 

 he sees no objections to be made, the illumination of the full moon is a little 

 less than the eight hundred thousandth part of the full illumination of the 

 sun. 



To complete the data of our definite calculation, we shall call to mind, 

 that, according to the density of the air in the lower strata of the atmos- 

 phere and its total weight, as indicated by the barometric column, the whole 

 stratum of air which constitutes the atmosphere is equivalent to a stratum of 

 about eight kilometres in thickness, and possessing the density of the air at 

 the surface of the earth. 



"We have already found that it would be necessaiy to render the comet 

 3GOO times more luminous for it to extinguish the lustre of a star of the 

 eleventh magnitude. To render a star of the fifth magnitude invisible it 

 would require to be made 3600+250 times more brilliant than it is. In 

 other words, if the atmosphcra were SGOO -f- 250 times less compact than it 

 is, it would be equivalent to the comet. As 3600 -f- 250 make 900,000, the 

 nine hundred thousandth part of the atmosphere would suffice to produce 

 the same effect of illumination as the comet ; but as the latter is in the full 

 light of the sun, while the atmosphere is only illuminated by the moon, 

 when it extinguishes stars of the fifth magnitude ; this circumstance gives 

 the atmosphere a further advantage in the proportion of 800,000 to 1, which 

 under ordinary circumstances gives the atmosphere a superiority equal to 

 900,000 -f- 800,000, or 720 billions. But this is not all ; the thickness of the 

 cometary substance being 500,000 kilometres, whilst that of the atmosphere 

 is only eight kilometres ; we must increase the above relation in the propor- 

 tion of 500,000 to 8, which brings it to forty-five millions of billions, thus- 

 45,000,000,000,000,000. 



Thus, according to these data, the density of the substance of a comet 

 could not be calculated at so high a quantity as that of the atmosphere, di- 

 minished by the enormous divisor, forty-five millions of billions. The shock 

 of a substance so rarefied would be nothing at all, and not the least particle 

 of it could penetrate even into the most rarefied parts of our atmosphere. 



According to experiments of my own, gases lose their property of elas- 

 ticitv Ion"- before they are reduced to such low density. I do not think that 



*/ * 



at the ordinary pressure a gas could completely fill a vessel with 20,000 times 

 the original volume of the gas. The substance of comets is, therefore, a 

 kind of very divided matter, with its molecules isolated and destitute of mu- 

 tual clastic reaction. 



It follows from the preceding that both the mass and the density of a 

 comet arc infinitely small, and without any hypothesis we may say that a 

 sheet of common air of one millimetre in thickness, if transported into the 

 region of a comet, and illuminated by the sun, would be far more brilliant 

 than the comet. 



