864 THE BACTERIOLOGICAL EXAMINATION OF AIR 



a smooth uniform layer on the lower surface of the tube not deep enough to reach 

 the orifice of the small glass tube nor the opening in the india-rubber at the other 

 end. [It will be found more satisfactory to warm the tube before pouring the 

 gelatin into it and then holding it horizontally to rotate the tube until the gelatin 

 has set so that the medium forms a thin coating over the whole of the interior.] 

 The apparatus is now ready for use. When about to carry out an experiment, 

 remove the outer piece of india-rubber, attach the outer end of the small glass tube 

 to an aspirator and draw 10-15 litres of air slowly through the tube. The air enters 

 the hole in the india-rubber capsule and passes over the surface of the gelatin on which 

 it is intended that the suspended dust should be deposited. When the required 

 volume of air has been drawn through, the outer india-rubber capsule is replaced 

 and the tube incubated in the cool incubator (20 C.). Colonies begin to appear 

 on the gelatin in the course of a day or two and if the technique was satisfactory 

 should be more numerous at the end at which the air entered. The colonies can 

 be counted and any which it may be desired to investigate further, picked off. 



If, for example, 15 litres of air have been aspirated and 6 colonies of bacteria and 10 

 moulds are subsequently counted on the gelatin the air will have contained approximately 

 T \ x 1000 = 400 aerobic bacteria per cubic metre. 

 |2 x 1000 = 666 moulds per cubic metre. 



In practice, however, it happens that many organisms stick to the glass wall of 

 thtf tube and so do not enter into the computation [this source of error is avoided if 

 the medium be coated over the whole surface] : further, if the experiment be con- 

 tinued for any length of time the gelatin will become dry and fail to act as a satis- 

 factory culture medium and lastly, the current of air must pass very slowly other- 

 wise the organisms suspended in it will be carried through the tube without being 

 deposited. [Two other objections may be raised, namely the difficulty of reaching 

 the colonies should it be desirable to sub-cultivate them and the fact that some 

 organisms rapidly liquefy the gelatin and render the experiment useless.] 



2. Methods employed at the present day. 



The methods just described have now been superseded by others which 

 depend upon removing the organisms contained in the air either by bubbling 

 the latter through a viscous fluid or by filtering it through a powder. By 

 adopting either of these methods all the organisms suspended in a given 

 volume of air can be collected in a small space, being either disseminated 

 in the liquid or mixed with the powder as the case may be. It will then only 

 be necessary to proceed on the lines already laid down in the sections dealing 

 with the isolation of organisms and with the examination of water. It is 

 always well to sow both agar and gelatin plates since the latter generally 

 liquefy in a short space of time. 



In carrying out these experiments with air some form of aspirator is 

 necessary. For choice, a water aspirator would be used such as is to be 

 found in chemical laboratories. With the aid of this apparatus the volume 

 of air aspirated can be very accurately measured. An ordinary water exhaust 

 pump can also be used. In this case it will of course be necessary to interpose 

 between the liquid through which the air is to bubble and the pump a gaso- 

 meter which will record the volume of air aspirated. [A still more simple 

 and quite satisfactory method consists in using a large glass barrel fitted with 

 a tap below and stoppered above with an india-rubber plug through which a 

 narrow piece of glass tubing is passed, such as is used in operating theatres 

 for storing antiseptics. This vessel can be graduated once for all by pouring 

 in measured volumes of water and marking the level on the glass with a 

 carburundum pencil. If it is required to aspirate, say, 10 litres of air, the 

 vessel is filled with water up to the 10 litre mark, the apparatus is then 

 attached to the small glass tube above and by regulating the flow of water 

 from the tap below the air can be aspirated at any speed which is considered 

 desirable. For the examination of air in places where it is inconvenient to 



