98 



BACTERIOLOGY 



small glass plate, -which is smeared with glycerine. The 

 aspirator being put in action, air streams in through the 

 upper aperture and deposits the greater part of the dust it 

 contains upon the glycerine, and the preparation is removed 

 from the cylinder and examined as soon as sufficient air has 

 been drawn through. The dust is distributed as evenly as 

 possible through the glycerine by stirring with a sterilised 

 steel needle, and the glass plate is covered with a second and 

 brought under the microscope. To calculate the amount of 

 dust in a litre of air, the particles in several microscopic fields 

 are counted, so as to ascertain the average number in each ; 



Glass head closed with 

 cotton wool 



Tube connected with 

 the aspirator 



Griass flask 



Fig-. 29. — MiQUEL's Apparatus for examining Air. 



from this the number spread over the whole plate is calcu- 

 lated, and thence the amount contained in a litre. 



Instead of the glycerine plate one of gelatine or agar 

 may be laid on the little table, and an attempt thus made 

 to isolate the micro-organisms (fig. 28). 



Miquel's method. — Miquel constructed a flask with two 

 lateral tubes (fig. 29) and another fitting by a ground joint 

 into the aperture at the top, and supporting a cap or head of 

 glass closed with a cotton-wool plug. One of the lateral 

 tubes is connected with an aspirator, the other (by means of 

 a piece of rubber pipmg) with a narrow glass tube sealed at 

 one end. The flask is filled with 30 to 40 c.cm. water, and 

 sterilised in the steam current ; the glass cap is then taken 

 ofi; and a given volume of air aspirated through, after which 



