BACTERIA IN AIR, EARTH, AND WATER. 385 



cultivated. Hesse's apparatus, for the estimation of the number and nature 

 of bacteria, c. , in the air, consists of a glass tube of about 18 to 25 inches 

 long, and ij inches in diameter, slightly funnel-shaped at each end ; in one 

 end an indiarubber bung is fixed. This has a central opening through 

 which passes a short glass tube about 5 or 6 inches in length, and one 

 third of an inch in diameter. In this short tube is inserted a plug of cotton 

 wadding. At the other end of the larger tube are two membranes of india- 

 rubber tied on separately ; the inner one has a perforation of about one-third 

 of an inch in diameter, the outer one fits over this and so closes the opening. 

 The tube, bung, and indiarubber membranes are then sterilized with a one 

 per cent, solution of bichloride of mercury and rinsed with boiled distilled 

 water. The inner cap is firmly tied in position with good stout thread, 

 the tube is half filled with water, and with a pair of scissors a hole is 

 clipped in the centre of the indiarubber membrane. The second cap 

 is then firmly tied on, water is again poured into the tube, the bung is 

 replaced, and the whole apparatus is thoroughly boiled for a quarter of an 

 hour in the steam sterilizer. After allowing time for the glass to cool, the 

 bung is removed, the water is carefully poured out and liquid nutrient 

 gelatine is poured in to replace it ; the bung is again fixed in position and 

 the apparatus is boiled for ten minutes at 100 C, after which the tube is 

 put in a cool place in a horizontal position, so that the gelatine may con- 

 solidate in a thin layer along the whole length of the tube. Just as the 

 gelatine is beginning to "set," the tube may be gently rocked from side to 

 side so as to obtain a rather larger cultivating gelatine surface. This tube, 

 so prepared, is placed in a V-shaped support resting on a tripod, where it is 

 held in position by a couple of elastic bands. To this tripod an aspirating 

 apparatus is fixed. This consists of two bottles, each fitted with corks 

 and tubes like those of a wash bottle. These bottles are placed at different 

 levels and are connected by an indiarubber band, in the middle of which 

 is a pinch cock. The indiarubber tube connects the longer tubes in the 

 flasks. A litre of water is then measured into the upper flask, and, by 

 applying suction to the orifice of the lower flask that is now left free, 

 water commences to flow from the upper into the lower flask. If now the 

 second tube in the upper flask be attached to the little tube projecting from 

 the bung of the tube containing the gelatine, the aspirating apparatus will 

 communicate with the chamber in which the gelatine is contained ; the 

 outer layer of indiarubber membrane which covers the orifice in the inner 

 membrane is then removed, and by setting the syphon system in operation 

 water is slowly and regularly run from the one flask into the other, and a 

 corresponding volume of air is drawn in at the small opening in the india- 

 rubber membrane. As soon as one litre of air has been drawn through, 

 all that is necessary is to close the pinch cock, reverse the positions of the 

 bottles, and repeat the whole process. This may be done until a volume of 

 from i to 5 litres of air has been drawn into and through the tube containing 

 the gelatine, the micro-organisms, drawn in along with this air, settling on 

 the nutrient medium in the tube. If the estimation is to be made in the 

 open air, it may be necessary to draw through the tube 20 or 30 litres. 

 It is, however, usually necessary to expose a number of tubes, drawing 

 different quantities of air through each ; then by taking an average of 

 those in which the organisms are most readily counted, a fairly accurate 

 result is obtained. The flow of water, and consequently the rate of flow of 

 air, may be regulated by the introduction of pieces of glass tubing of known 



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