128 



BACTERIA IN AIR 



all the organisms will be found to have fallen out of the air on to the 

 gelatin in the course of their transit. This fact can be tested by 

 interposing between the tube a and the aspirator a second tube prepared 

 in the same way, which ought, of course, to show no growth. When 

 forty-eight hours at 20 C. or four days at lower temperature have 

 elapsed, the colonies which develop in a may be counted. The dis- 

 advantage of the method is that if particles of dust carrying more than 

 one bacterium alight on the gelatin, these bacteria develop in one colony, 

 and thus the enumeration results may be too low ; difficulties may also 

 arise from liquefying colonies developing in the upper parts of the tubS 

 and running over the gelatin. 



Petri's Sand-Filter Method. A glass tube open at both ends, and 

 about 3| inches long and half an inch wide, is taken, and in its centre is 

 placed a transverse diaphragm of very fine iron 

 gauze (Fig. 51, e) ; on each side of this is placed 

 some fine quartz sand which has been well washed, 

 dried, and burned to remove all impurities, and this 

 is kept in position by cotton plugs. The whole is 

 sterilised by dry heat. One plug is removed and a 

 sterile rubber cork, c, inserted, through which a tube, 

 d, passes to an exhausting apparatus. The tube is 

 then clamped in an upright position in the atmo- 

 sphere to be examined, with the remaining plug, 

 /, uppermost. The latter is removed and the air 

 sucked through. Difficulty may be experienced 

 from the resistance of the sand if quick filtration be 

 attempted. The best means to adopt is to use an 

 air-pump the amount of air drawn per stroke of 

 which is accurately known and have a manometer 

 (as in Fig. 31) interposed between the tube and the 

 pump. Between each two strokes of the air-pump 

 the mercury is allowed to return to zero. After 

 the required amount of air has passed, the sand a 

 is removed, and is distributed among a number of 

 sterile gelatin tubes which are well shaken ; plate 

 cultures are then made, and when growth has occurred 

 the colonies are enumerated ; the sand b is similarly 

 treated and acts as a control. 



When it is necessary to examine air for particular 

 organisms, special methods must often be adopted. 

 Thus in the case of the suspected presence of tubercle bacilli a given 

 quantity of air is drawn through a small quantity of water and then 

 injected" into a guinea-pig. 



It must be admitted that comparatively little information 

 bearing on the harmlessness or harmfulness of the air is obtain- 

 able by the mere enumeration of the living organisms present, 

 for under certain conditions the number may be increased by 

 the presence of many individuals of a purely non-pathogenic 

 character. The organisms found in the air belong to two 

 groups firstly, a great variety of bacteria ; secondly, yeasts and 

 the spores of moulds and of the lower fungi. With regard to 

 the spores, the organisms from which they are derived often 



FIG. 51. Petri's 

 sand filter. 



