COMETARY INFLUENCES. 



tails. These appendages where they exist are generally of very 

 limited length, but in some rare instances, as has been already 

 stated, their length is prodigious, extending over a space not less 

 than the thirtieth part of the extreme diameter of the solar system. 

 If such a comet had its head at the surface of the sun and its tail 

 in the plane of the ecliptic, the tail would sweep over the space 

 through which the planets, Mercury, Venus, the Earth and Mars 

 move, and it might in that case encounter any or all of these 

 planets. 



It cannot, therefore, be denied that the immersion of the earth 

 in the tail of a comet is a possible event. That it is extremely 

 improbable, however, may be shows, by the same reasoning as 

 has been already stated in reference to the question of the pro- 

 bability of the collision of a comet and the earth, combined with 

 the consideration that very few comets have tails of considerable 

 length. 



But, supposing such an event to take place, what would be the 

 probable consequences ? 



It is certain that the matter composing the tails of comets is of 

 such a nature that although these appendages have often a thick- 

 ness measuring many thousand miles, the smallest telescopic stars 

 are visible through it, without the least perceptible diminution of 

 their lustre. 



The matter of the tail being, therefore, so completely trans- 

 parent, and producing moreover no perceptible refraction, its 

 density, if it be vaporous or aeriform, must be extremely incon- 

 siderable, and according to all probability, many thousands of 

 times less than the density of our atmosphere. 



If such be its nature, when the earth would pass through it, it 

 would mingle with the terrestrial atmosphere, and if its density 

 were, for example, a thousand times less dense than the air, the 

 atmosphere would contain one particle of cometic matter to every 

 thousand particles of pure air. 



Let us suppose that the room we inhabit contains 10,000 cubic 

 feet of air, and let 10 cubit feet of any noxious gas be introduced 

 into it and mixed with the air. We should then take into the 

 lungs in respiration one particle of the noxious gas with every 

 thousand particles of pure air. So far as the possible injurious 

 effects depend on the numerical proportion of impurity, there 

 would appear in such case to be but little ground of reasonable 

 fear. 



"We have, however, numberless examples of the strong effect 

 produced upon our organs by effluvia with which the air is occa- 

 sionally impregnated, which, nevertheless, prevail in a proportion 

 so minute as utterly to escape the nicest and most exact analysis. 

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