It is acknowledged by most pathologists that tubercular sputum, dried up and broken into dust, is the most common vehicle by which the bacillus of tubercle is conveyed into the body. But its power for evil is obviously modified by a multitude of conditions, some of them inherent in the animal body exposed to infection, others due to external influences. Judging from the facts relating to the distribution of tubercular disease, its incidence in certain localities, and especially its prevalence in badly drained, badly ventilated, and imperfectly lighted dwellings, it has been surmised that the three chief external conditions that mitigate the virulence of the bacillus are: (1) a dry soil (2) abundance of fresh air, and (3) light.