214 AIDS TO BACTERIOLOGY 



cavity, which can be closed up by turning a handle one 

 way, or opened by turning in the opposite direction. The 

 borer is pushed down to the necessary depth; the handle 

 is then turned; the earth enters the cavity; the handle is 

 again turned, enclosing the sample of earth completely; 

 the borer is then withdrawn. The soil is thoroughly 

 mixed with melted nutrient gelatin, which can be poured 

 into a Petri dish, or, better, made into a roll culture by 

 Esmarch's method. Another method is to wash the soil 

 with sterile water, which is examined, as usual, by the 

 plate method. 



The existence of faecal pollution, and whether it is recent 

 or distant, is regarded by Houston as being shown by the 

 following factors: Total number of aerobic organisms; 

 number of spores present; estimation of B. coli, B. Welchii 

 and streptococci. Houston found -that streptococci dis- 

 appear rapidly, and regards them as indicating very 

 recent pollution; the spores of B. V/elchii, on the other 

 hand, persist for longer periods. 



Tetanus and malignant oedema bacilli are detected by 

 inoculating glucose formate broth, heating to 80 C. in 

 a water-bath for twenty minutes, and then cultivating 

 anaerobically. 



The Bacteriology of Air. 



No particular organisms can be regarded as character- 

 istic of air. Those that do occur are derived from dry 

 surfaces, and are carried with the dust by air-currents. 

 A diminution in numbers occurs with an increase in alti- 

 tude, with an increase of distance from towns into country, 

 and in mid- ocean the air is nearly sterile. The species 

 generally found are spores of moulds, yeasts, bacterial 

 spores, and chromogenic bacteria. Owing to the influence 

 of desiccation, B. coli is seldom met with. The presence 

 of tubercle bacilli in dust has been abundantly proved. 

 In sputa found on the ground tubercle bacilli were present 

 in 60 per cent. Examinations of sewer air have given 

 discordant results. Some find half the number of organisms 

 present in external air, and that organisms normal 1o 

 sewage are comparatively rare. Andre wes found strepto- 

 cocci, generally corresponding with the S. salivarius 

 type, while the S. equinu-s type is most abundant in fresh 

 London air. He also found numerous B. coli. Horrocks 



