Besides the contagia of the acute fevers, _septic organisms_ may be carried by the atmosphere. Formerly, blood-poisoning from operation and other wounds was common; but Lister, by insisting on absolute cleanliness of wounds, and only allowing air to have access to the wound which had been filtered through layers of gauze and deprived of its septic germs, has secured that wounds can now be kept perfectly “sweet,” the suppuration in them reduced to a minimum, and the danger of blood-poisoning almost annihilated (page 106). It had often been noticed that recovery from even very severe injuries was common, if only the skin remained unbroken; while the same injuries, with the addition of a rupture of the skin, and consequent access of air, were rapidly fatal. But to Lister is due the great honour of proving that it was not the air which produced the mischief, but the germs it contained, and that filtered air might be admitted with impunity. _Erysipelas_ and _hospital gangrene_ have occasionally been carried about in hospital wards by dirty sponges and dressings; and if the ventilation is not perfect, particles of epithelium and pus from diseased persons may be carried to other patients at a distance. Some forms of _purulent disease of the eyes_ are transferable from patient to patient, and in children some forms of _eczema_ are also contagious. CHAPTER XVI. GASEOUS AND OTHER IMPURITIES OF AIR. Gaseous impurities of the air are very commonly associated with suspended matters, and it is often difficult to separate the effects of the two. Different gases are also often associated, and so produce modified results. It will be convenient to consider, first of all, certain well-marked gaseous impurities, and then others in which there is a mixture of several gases, or of these with suspended solid particles. Under the first head the most important impurity is— (1) =Carbonic Acid.=—This is reckoned an impurity if amounting to more than 5 parts in 10,000 of air. Owing to the large amount produced in the respiration of animals, in the combustion of fires, gas, lamps, etc., and in other natural processes, it would be much greater in populous parts, were it not for the rapid diffusion occurring in the air, and the purifying action of plants. The following analyses (Angus Smith) illustrate the facts that in towns the amount rises, and is greatest in the most populous parts, while during fogs it is greatly increased.