Instances are likewise known where graves containing human bodies, long dead of plague, upon being opened, have emitted effluvia, which have produced typhus fever among the workmen. It is probable that, in general, the effluvia arising from dead animal materials, undergoing decomposition in the ordinary way, are the common results of the putrefactive fermentation,—carbonic acid gas, hydrogen, nitrogen, &c. These gases form various combinations; carbonic acid gas and hydrogen gas forming carburetted hydrogen, an inflammable gas, the same as is used for the purpose of illumination, and which cannot support respiration. Hydrogen unites with nitrogen, and forms ammonia, or spirit of hartshorn, which is volatile, and imparts a strong odour to the atmosphere, such as is experienced in stables and byres, producing sneezing and watering of the eyes. Hydrogen, at its extrication, sometimes carries with it a portion of phosphorus, already contained in the decomposing body, and becomes phosphuretted hydrogen, a gas which ignites spontaneously in the atmosphere, the same that is sometimes observed in churchyards under the title of corpse-lights. Hydrogen sometimes also unites with sulphur, and the combination is called sulphuretted hydrogen, a gas readily discovered by its offensive odour,—which it imparts to many very useful mineral waters. These gases are discovered, not only in an atmosphere exposed to decomposing dead animal materials, but are also found in that atmosphere containing numbers of men in health, closely crowded together, and persons suffering putrid diseases, where no attention is paid to cleanliness and the removal of impurities. A body affected with putrid disease is more liable to decomposition than one in health; and the secretions and excretions are more prone to putrefaction, and the emission of effluvia or gases. Some facts are known, which shew that bodies, in some forms of low or malignant disease, both before and after death, possess a virulence, never found in bodies in health, or affected with disease of a non-malignant character. The worst consequences have followed wounds in the dissection of bodies recently dead of typhus fever; the introduction, under the skin, of the fluid contained in the petechiæ or black spots common in that disease, and even the washing of bandages and clothes employed in cases of mortification and the like. In such diseases, the body becomes a very centre of contamination and virulence; its fluids become acrid and poisonous; and on the surface of the body, fluids are elaborated, which are productive of the most malignant and pestiferous effects. Whether these fluids, those virulent secretions, are ever diffused in the air, and impart to it their deadly properties, is a point of much interest, but one which cannot be entertained here.