Illuminating gas, as we all know, is poisonous. If a Comet’s tail were dense enough, it is conceivable, therefore, that every human being on this planet might be asphyxiated by breathing the Comet’s poisonous vapour as the Earth plowed through it. There is also this possibility, suggested by Flammarion, that the gases of a very dense tail might so combine with the nitrogen which constitutes nearly 80 per cent. of the air we breathe, that the atmosphere would be converted into the “laughing gas” employed by dentists. The world would die in a delirium of joy. At first a delightful serenity would settle upon mankind. Then would follow a contagious gaiety, febrile exaltation, a paroxysm of delight, and then madness. Flammarion even conceives the world merrily dancing a joyous, hysterical sarabande in which it perishes laughing. The tail of a Comet is fraught with still other possible dangers. Our atmosphere contains a certain amount of hydrogen, a marvellously light gas to which balloons owe their buoyancy. Besides its lightness, this gas is characterized by an extreme inflammability. The law of the diffusion of gases teaches us that part of this hydrogen in the air is mechanically mixed with other gases, and that part of it probably floats in the upper air, far beyond the reach of any balloon. A Comet may be regarded as a huge lighted torch whirling through space, which may be brought dangerously near that upper layer of highly inflammable hydrogen. If the gas shall ever be touched off by this flying torch, our planet will be ignited. The whole atmosphere will become a seething ocean of flame, in which forests and cities will burn like straw, in which oceans will boil away in vast clouds of steam, and in which all animal life will be snuffed out of existence before it shall realize that the world is on fire. In a word, the globe will become a planetary funeral pyre. Since water results from burning hydrogen in oxygen, this same fierce and terrible flame must be speedily extinguished by a mighty deluge which will engulf the Earth. A spectroscope analysis of Halley’s Comet has furthermore revealed the presence of cyanogen gas in the tail. Cyanogen is a compound of nitrogen and carbon, one of the most poisonous compounds with which the chemist is familiar. Prussic acid, potassium cyanide and many other cyanides, all of them almost instantaneously fatal if taken into the human system, are compounds of cyanogen. If that gas is present in large enough quantities, one flick of a Comet’s tail will end all human and animal existence.