376 CELESTIAL CHEMISTRY. 



this case. These resuUs were not confined to the nucletis of 

 the comet only, but extended for some distance along' the tail. 



Camille Flammarion recently made some rather sensational 

 statements regarding what might happen in the event of a 

 •cometary tail containing cyanogen becoming diffused through 

 the earth's atmosphere, but it would seem that the probabilities 

 of the human race being poisoned by hydrocyanic acid in 

 such a case are about as slight as the chances of our being 

 deluged with paraffin from the hydrocarbons in the comet's tail. 

 We see the lines of these substances in the comet's spectrum, 

 it is true, but then it must be remembered, we are looking trans- 

 versely though tens of thousands of miles thickness of tail, and 

 .the materials of which that tail is composed are so widely scat- 

 tered that they would make no perceptible difference to our 

 .atmosphere were they to become mixed up in it; indeed in 1861 

 the earth actually passed right through the tail of a comet and 

 was immersed in that tail to a depth of 30,000 miles without any- 

 thing untoward happening, and only a few astronomers detected 

 .the luminous yellow haze in the sky. 



Some comets, amongst them the great comet of 1882, have 

 ibeen found to contain sodium vapour, but in this case, as also in 

 that of Wells's comet, this particular phenomenon appeared to 

 occur only when the comet got into close proximity to the sun, 

 and the soditmi became volatilised by the great heat there ex- 

 perienced. 



It must be remembered that comets pass through very great 

 ^extremes of temperature; the comet of 1811, with its orbit of 

 40,000 million miles diameter, which it takes over 3,000 years to 

 traverse, passes from a temperature of 250 degs. below zero 

 to that involved in close quarters to a sun whose iron is vapour. 

 At its furthest distance from the sun, and its extreme of cold, 

 such a comet probably is almost wholly solid, and the space 

 which it occupies is small ; but as it approaches the solar heat it 

 gradually vapourises, and evidences, not only of sodiimi, but 

 also of iron and magnesium become apparent in the spectrum, 

 whilst its mantle of hydrocarbons — paraffins or benzenes, maybe 

 — scarcely visible at first, breaks up chemically into hydrogen 

 and free carbon — multitudes of finest particles of soot, which 

 reflect the sunlight, and under the action of the mechanical 

 pressure of the sun's rays, these particles are driven as by a re- 

 pulsive force outwards from the nucleus and from the central 

 luminary as a tail, until they stretch over millions upon millions 

 of miles. In this way the tail of the great comet of 1680 darted 

 forth to a length of over 60 million miles in two days. If these 

 particles vary in size the tail will of course be curved, and if 

 thev consist of different chemical compounds, their constituents 

 will break up one after another, and as the light-pressure gets 

 an opportttnity of acting on one after another, successive tails 

 will be formed with varying degrees of curvature. 



Incredible though it may seem, the gases present in comets 

 have actually been handled and analysed in chemical laboratories 

 on this very earth; the gases occluded in meteorites which 

 have fallen on this earth have been found to be identical with 

 tli3?e spectroscopically detected in comets, viz., hydrogen, carbon 



