As we have so lately had occasion to refer to that virulent epidemic, the yellow fever, we cannot but be doing service in giving some information concerning the above, the result of investigations conducted by an English gentleman, and communicated by him to the London Engineer. There is little doubt that epidemic and putrid diseases owe their origin, in the first instance, to a product of putrefactive fermentation, or that change which goes on in the decay of animal and vegetable substances. Whether this product be a new compound alkaline body, or an organic germ, the one of far greater power than any alkaloid at present known, the other rapidly producing a morbid change in the animal system, or a powerful ferment, volatilized, and carried off by other bodies; its fearful consequences cannot but excite our wonder, and its existence should excite the man of science to study its properties, so that we may place it out of the domain of conjecture into the realms of certainty. By specifying a few examples of the action of " putrefactive poison," we shall be able to more correctly ascertain its nature. The malaria of India, Ceylon, and the Campagna of Rome, the subtle emanations which occasionally follow the course of rivers, the effluvia of marshes, cesspools and drains, and the poisons generated in certain preparations in animal food seem to be but modifications of the same cause. If so, the study of one will afford a key to the whole. Our authority has chosen the malaria of Ceylon; let us follow him. This emanation is supposed to owe its virulence to sulphureted hydrogen, but that gas when diluted with air does not produce the effects of malaria; so that it is no longer the gas of the laboratory, but has acquired new properties by contact ivith decaying vegetation. It holds in solution an organic poison, the composition of which analysis cannot reveal. By imitating this process in the laboratory, namely, passing sulphureted hydrogen through water containing putrefying vegetable matter, we find that it has a new odor and peculiar properties. When decomposed by chloride of lime it deposits carbon as well as sulphur, and it is probable that the remaining constituent of the poison is hydrogen. If this can be proved, the poison is a true hydrocarbon, and it is probable that it only emanates where there is no growing animal or vegetable life to assimilate it while in an innocuous state, because it has been observed that vegetable as well as infusorial growth lessens the dangerous char- acter of putrefactive poisons. The gases with which this poison assimilates itself, and on whose character much of its virulence depends are the following:— 1 Hydrogen, 2 Sulphureted hydrogen, 3 Carbureted hydrogen, 4 Phosphureted hydrogen. The study of the properties and peculiarities of these gases assumes therefore an importance which they never possessed before. Fungi are also known to propagate disease, but when we remember the extraordinary diffusive character of the gases, the growth of fungi as a means of spreading disease falls into insignificance. We think that this chemical theory is the key to the mystery, and it only now remains for some hero of science greater than Alexander, Caesar or Hannibal, to unlock the door, and make the grandest conquest the world has ever seen, namely, the conquest of man over epidemic disease.