348 TEXT-BOOK OF BACTERIOLOGY. 



The most primitive procedure is that with common plates 

 coated with gelatin, freely exposed for a certain time and then pre- 

 served and treated in the manner already familiar. The micro- 

 organisms that descended upon them will, in a few days, develop 

 into colonies whose numbers and kind will enable us to draw con- 

 clusions as to their character. 



Simple and convenient as this method may appear, it still lacks 

 perfection. Its most essential defect consists in the circumstance 

 that the quantity of air coming 1 in contact with the nourishing 

 medium can by no means be carefully estimated and that there is 

 no certainty that all germs capable of development have actually 

 been deposited. The celerity of motion of the air varies so greatly 

 every moment, that comparable results cannot at all be reached in 

 this way. 



This defect was remedied to a certain extent by a procedure 

 devised by Koch after the introduction of solid transparent media. 

 The flat glass dish destined to receive the gelatin is I cm. high 

 and has a diameter of 5 cm. ; it is placed at the bottom of a cylin- 

 drical glass vessel 6 cm. in diameter and 18 cm. high. 



This glass dish can be easily lifted from the cylindrical vessel by 

 means of a narrow strip of sheet iron bent into a right angle, 

 for the purpose of future microscopical examination of colonies. 

 The glass is now closed with a solid, large plug of wadding and the 

 whole apparatus is sterilized in the hot-air oven. The plug is then 

 raised, the cup taken out, filled with gelatin and at once lowered 

 again and closed anew by the plug. 



The gelatin having become hard, the plug is removed at the 

 spot where the air is to be examined and carefully preserved, while 

 the apparatus remains open for a certain number of hours. The 

 strata of air in the glass- vessel may now be considered as in repose, 

 i.e., we may count on approximately equal quantities of air de- 

 positing their germs upon the gelatin within a given time. When- 

 ever the germs have developed to colonies, the apparatus is re- 

 opened, the gelatin-glass is lifted to the surface and at once exam- 

 ined under the microscope. 



An exact examination of quantities of air can, it is true, not be 

 obtained in this way. The procedure was decidedly improved by 

 the work of Hesse. 



Hesse's method of investigating air is essentially as follows: 



A glass tube, about 70 cm. long and 4 cm. wide, is provided at 

 one end with a thick rubber stopper with a central perforation for 

 the reception of a little glass tube, 1 cm. wide and 10 cm. long, 

 stopped at each end by a dense plug of wadding. The other aperture 



