into a guinea-pig produced tetanus.[29] The presence of pathogenic bacteria in the air is, of course, a much rarer contamination than the ordinary saprophytes. Tubercle has been not infrequently isolated from dry dust in consumption hospitals, and in exit ventilating shafts at Brompton the bacillus has been found. From dried sputum it has, of course, been many times isolated, even after months of desiccation. M. Lalesque failed to isolate it from the dry soil surrounding some garden seats in a locality frequented by phthisical patients. The writer also failed to isolate it from the same soil. But a very large mass of experimental evidence attests the fact that the air in proximity to dried tubercular sputum or discharges may contain the specific bacillus of the disease. Diphtheria in the same way, but in a lesser degree, may be isolated from the air, and from the nasal mucous membrane of nurses, attendants, and patients in a ward set apart for the treatment of the disease. Delalivesse, examining the air of wards at Lille, found that the contained bacteria varied more or less directly with the amount of floating matter, and depended also upon the vibration set up by persons passing through the ward and the heavy traffic in granite-paved streets adjoining. _Bacillus coli_, staphylococci, and streptococci, as well as _B. tuberculosis_, were isolated by this observer. Some new light has been thrown upon the subject of pathogenic organisms in air by Neisser in his investigations concerning the amount and rate of air-currents necessary to convey certain species through the atmosphere. He states that the bacteria causing diphtheria, typhoid fever, plague, cholera, and pneumonia, and possibly the common _Streptococcus pyogenes_, are incapable of being carried by the molecules of atmospheric dust which the ordinary insensible currents of air can support, whilst _Bacillus anthracis_, _B. pyocyaneus_, and the bacillus of tubercle are capable of being aërially conveyed. This work will require further confirmation, but if its truth be established, it proves that attempted aërial disinfection of the first group of diseases is useless. CHAPTER IV _BACTERIA AND FERMENTATION_