In view of the proved possibility of infectious matter being carried by air, reasonable efforts to free air supplies from living micro-organisms are justified in the light of general sanitary principles. The great reduction of intestinal disease through water purification since the turn of the century might prompt us to hope that some of the diseases transmitted through discharges from the respiratory tract may be checked by methods of controlling air supplies. A study of the effect of ventilating factors on the dispersion and viability of micro-organisms suspended in air20has already disclosed the efficacy of certain destructive agencies. Experiments on the disappearance of B. coli in the recirculated air of an air-conditioned room were conducted as a preliminary to the investigation of textile mill humidification referred to previously.12B. coli seemed to disappear much more rapidly from air when the spray humidifier was operating than when the