78 BACTERIA IN THE AIR 



measured in the usual way. Sterilised water was then added to bring 

 the glycerine to a known volume, the liquid thoroughly mixed, and a 

 series of gelatine and agar plates made with quantities varying from 

 01 to 2 c.c. By this method a large number of bacteria were 

 detected in this particular investigation, including Staphylococcus 

 pyogenes auretis and alius, the common Bacillus suhtilis, and, appar- 

 ently, B. coli communis* Carnelly, Haldane, and Anderson found 

 11 bacteria per litre of air in a classroom of the High School at 

 Dundee with the boys at rest. But when the boys were instructed 

 to stamp on the floor, and thus raise the dust, the number rose to 150 

 bacteria per litre. 



During a six years' investigation the air of the Mont Souris 

 Park yielded, according to Miquel, an average of 455 bacteria per 

 cubic metre. In the middle of Paris the average per cubic metre 

 was nearly 4000. Pliigge accepts 100 bacteria per cubic metre as a 

 fair average. From this fact he estimates that " a man during a life- 

 time of seventy years inspires about 25,000,000 bacteria, the number 

 contained in a quarter of a litre of fresh milk."-]- Many authorities 

 would place the average much below 100 per cubic metre, but even 

 if we accept that figure it is at once clear how relatively small it is. 

 This comparative freedom from bacteria is due to sunlight, raia, 

 desiccation, dilution of air, moist surfaces, etc. So essentially does 

 the bacterial content of air depend upon the facility with which 

 certain bacteria withstand drying that Dr Eduardo G-ermano has 

 addressed himself first to drying various pathogenic species and then 

 to mixing the dried residue with sterilised dust and observing to 

 what degree the air becomes inf ected.j The typhoid bacillus appears 

 to withstand comparatively little desiccation, without losing its viru- 

 lence. Nevertheless it is able to retain vitality in a semi-dried con- 

 dition, and it is owing to this circumstance, in all probability, that it 

 possesses such power of infection. The bacillus of diphtheria, on the 

 other hand, is capable of lengthened survival outside the body, 

 particularly when surrounded by dust. The question of its power of 

 resistance to long drying is an unsettled point. The power of 

 surviving a drying process is, according to Germane, possessed by the 

 Streptococcus pyogenes. This is not the case with the organisms of 

 cholera or plague. Dr G-ermano classifies bacteria, as a result of his 

 researches, into three groups : first, those like the bacilli of plague, 

 typhoid, and cholera, which cannot survive drying for more than a 

 few hours ; second, those like the bacilli of diphtheria and strepto- 

 cocci, which can withstand it for a longer period ; thirdly, those like 

 the tubercle bacillus, which can very readily resist drying for months 



* Pufiiic ifeaZ^/j, vol. X., No. 4, p. 130 (1898). 



t Fliigge, Grundriss der Hyyieiie, 1897, pp. 161, 162. 



J Zdtschrift fiir Hygiene, vols, xxiv.-xxvi. 



