108 BACTERIA 



glycerine. The air was slowly drawn through and 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 o. I to 2 cc. By this method a large 

 number of bacteria were detected in this particular invest- 

 igation, including Staphylococcus pyogenes aureus et albus, 

 the common Bacillus subtilis, and B. coli communis. 1 



During a six years' investigation the air of the Montsouris 

 Park yielded, according to Miquel, an average of 455 bac- 

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

 per cubic metre was nearly 4000. Fliigge accepts 100 bac- 

 teria per cubic metre as a fair average. From this fact he 

 estimates that " a man during a lifetime of seventy years in- 

 spires about 25,000,000 bacteria, the same 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 is due, as we have mentioned, 

 to sunlight, rain, desiccation, dilution of air, moist surfaces, 

 etc. So essentially does the bacterial content of air depend 

 upon the facility with which certain bacteria withstand dry- 

 ing that Dr. Eduardo Germano 3 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 infected. Typhoid appears to with- 

 stand comparatively little dessication, without losing its 

 virulence. Nevertheless, it is able to retain vitality in a 

 semi-dried condition, and it is owing to this circumstance 

 in all probability that it possesses such power of infection. 

 Diphtheria, on the other hand, is, as we have pointed out, 

 capable of lengthened survival outside the body, particularly 



1 Public Health, vol. x., No. 4, p. 130 (1898). 



2 Fliigge, Grundriss der Hygiene, 1897. 



3 Zeitschrift fur Hygiene, vols. xxiv.-xxvi. 



