PETRT'S SAND-FILTER METHOD 



149 



i which develop in a may be counted. The disadvantage 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 

 numeration results may be too low ; difficulties may also arise from 

 liquefying colonies developing in the upper parts of the tube and running 

 over the gelatin. 



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

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

 placed a transverse diaphragm of very fine iron gauze 

 (Fig. 49, 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, r, inserted, through which a tube, d, passes to 

 an exhausting apparatus. The tube is then clamped 

 in an upright position in the atmosphere to be ex- 

 amined, 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 to have a manometer (as in Fig. 30) 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 arc 

 well shaken ; plate cultures are then made, and 

 when growth has occurred the colonies are enumer- 

 ated ; 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. 



d 



FIG. 49. Petri's 

 sand filter. 



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 

 consist of felted masses of threads, from which are thrust into 

 the air special filaments, and in connection with these the spores 

 are formed. By currents of air these latter can easily be detached. 



