dust is harmless. It is not so, however, with the dusts produced abundantly in various trades, such as in the manufacture of cutlery, pottery, porcelain, glass, copper, iron, steel, brass and lead wares, in stone-cutting and cotton manufacture. Some of these industrial dusts are poisonous; some are mechanically irritating to the walls of our air passages. In dusty occupations such diseases as bronchitis, tuberculosis and pneumonia are unduly prevalent, and there is no doubt that their beginnings lie in local injuries to the lungs produced by the inhaled particles and that these injuries are followed by the lodgment of the specific bacteria.
Of the bacteria of the air and their relation to disease I must speak at greater length. From the earliest times the belief has existed that bad air is a prolific source of disease. The word "malaria" literally means "bad air," and the disease malaria was the type of those diseases that were supposed to be spread through the atmosphere. In the early days of the germ theory air was regarded as the chief medium of the transmission of disease germs. As one writer graphically put it, disease is "literally borne on the wings of the wind." The great surgeon Lister accepted this notion and conceived the idea of improving surgical technique by maintaining a continual and very fine spray of carbolic acid in the air in the immediate vicinity of the operation. Thus antiseptic surgery arose. Although surgery has now gone far beyond this stage and no longer regards the air as a source of operative infection, the general notion of aerial infection still prevails. But a multitude of facts, gradually accumulated, show that this notion must be revised. It is true that bacteria may be moved through the air, and this may occur under three conditions: when they are freely floating, when they are attached to particles of dust, and when they are contained within the bodies of flying insects. The dissemination of disease germs by insects is a serious fact—the mysterious miasma of malaria lies only within the body of the mosquito, and malaria is still the type, but in a new sense, of certain diseases that are spread through the atmosphere. But there are many reasons for believing that the danger of infection through germs freely floating in air or attached to particles of dust has been much exaggerated. Living organisms, it is true, may be found in the atmosphere of inhabited localities under almost any circumstances. To capture them it is only necessary to expose to the air for a few moments a sterilized plate covered by a layer of nutrient agar on which the floating particles may fall. If the plate be then covered and transferred to a warm place, the organisms will proceed to multiply and develop colonies. It is then found that they comprise bacteria and some other microscopic forms. By far the greater number are quite harmless, but pathogenic or disease-producing species do occur. These may include germs of tuberculosis, diphtheria, typhoid fever, dysentery,
