io6 



EXAMINATION OF AIR AND WATER 



up into the pipette two or three times by aspiration, so as to 

 wash down into the tube any chance organisms adhering to its 

 inner walls. By careful tilting, the water is then caused to flow 

 by the horizontal tubulure into the second tube, washing the 

 tube walls as it passes. The water is then aspired gently two 

 or three times into the gauged pipette, after which any given 

 quantity can be removed for the purpose of plate cultivation. 

 The number of organisms per metre cube of 

 air is calculated according to the formula given 

 above. If the air taken is at any great dis- 

 tance from where the plate cultivations are to 

 be made, the tubes must be kept in ice during 

 transit in order to avoid 'the multiplication of 

 organisms. 



The above two methods are those which 

 have given the most satisfactory results in our 

 hands. 



4. Method of Strauss and Wurtz— 



In this method, which has been largely used 

 by Duclaux, the air is drawn through melted 

 nutrient gelatine in the special form of tube 

 shown in Fig. 32. 



This consists of a glass cylinder drawn to 

 a neck at the upper extremity, and terminating 

 at the lower with a closed contraction in the 

 tube of about 15 millimetres in diameter. 

 Ground into the neck so as to close it hermeti- 

 cally, is a hollow glass stopper of the form 

 shown. From its inferior surface this stopper 

 is continued down almost to the bottom of the 

 lower constriction in the tube, in the form of a 

 pipette calibrated in divisions of i cc, while the 

 superior surface is fitted as shown, and furnished with a cotton- 

 wool plug. The lateral branch is constricted in the centre, and a 

 plug of glass wool is placed in position on each side of the constric- 

 tion. The apparatus is then sterilised in the hot air steriliser at 

 1 50° C. for one hour. 



Ten cc. of liquefied gelatine are placed in the tube with the 

 usual precautions, one drop of sterile oil is dropped on to the 

 surface of this, and the whole is then sterilised, and subsequently 

 allowed to cool. When required for use, the gelatine is melted and 



Fig. 32. — Apparatus of 

 Strauss and Wurtz. 



