It has been often said that we cannot tell the difference between the air of the mountain-side and that of the crowded hospitals and fever-nests of towns. If it were so, it would be sufficient to say, Life is a more delicate test than Chemistry. But it is not so. The impurities in these pernicious places can be detected by chemical analysis, and examined as readily as the constituents of the atmosphere itself. Footnote 5: It has been alleged that the Cattle Plague owed its existence to these among perhaps other kindred causes, and human Epidemics have frequently been preceded or accompanied by a murrain among Cattle. See p. 7, and Boa Vista fever, _pot._ [ED.] The moisture in the air of a crowded room may be condensed by ice. It condenses indeed spontaneously on the walls and windows, and on all surfaces, and may be collected in sufficient quantity for examination and experiment. If a portion of this deposit be put on a piece of platinum and burnt, a strong odour of organic substance is given off, and a quantity of charcoal remains. If the deposit be allowed to stand for a few days, it forms a solid, thick, glutinous mass, having a strong odour of animal matter. If examined by a microscope, it is seen to undergo a remarkable change. First of all, it is converted into a vegetable growth, and this is followed by the production of multitudes of animalcules,—a decisive proof that it must contain organic matter, otherwise it could not nourish organic beings.[6] Footnote 6: See the interesting experiments of Dr Angus Smith, on the Air and Water of Towns, “Report of the British Association for the Advancement of Science,” p. 16, _et seq._ At every expiration the lungs pour a portion of organic matter into the surrounding atmosphere; at every moment the skin does the same. This matter is the dead portion of the body, which it is one of the special offices of these depurating organs to remove out of the living system as useless and pernicious. It is indeed pernicious, for it is an animal poison, more concentrated in this than in any other form of excrementitious matter, since in other excretions the noxious particles, in their transmission out of the body, are diluted with other substances, but as they issue from the lungs and skin, they are in a great degree undiluted. Ventilation and cleanliness prevent this matter from accumulating, and render it innoxious. But it collects in large quantities on the furniture and walls of dirty houses, and is the main cause of the disagreeable smell of the rooms in which it abounds. In some instances the walls are coated with it. It was so in one particular building in which, during a local epidemic outbreak, twelve persons were attacked with cholera, and four died.