EXAMINATION OF AIR AND WATER. 41 



extent, and their number continually diminishes as long as 

 the earth is moist ; but when the ground dries, it gradually 

 increases again. Thus in the dry seasons of the year the 

 number of bacteria is usually the greatest, whilst the mould- 

 fungi, which thrive best in moisture, and whose organs of 

 reproduction project upwards, are most abundant in the air 

 during the wet seasons. The purest air is found in the winter 

 time ; the air of towns is less pure than that of the country ; 

 germ-free, or nearly germ-free air is found at sea and on high 

 mountains. In certain localities hospitals, for instance the 

 air has been found to be very rich in bacteria ; in one case 

 even 50 times richer than the air in the garden at Montsouris. 



An entirely different method for the examination of the 

 organisms contained in air is that employed in KOCH'S 

 laboratory, and more completely developed by HESSE. A 

 glass tube, about 1 meter long and 4 to 5 cm. wide, is closed 

 at one end with a perforated india-rubber membrane, over 

 which another non-perforated cap is bound. A little liquid 

 nutritive gelatine is then poured into the tube, after which the 

 other end of the tube is closed with an india-rubber stopper, 

 through which passes a glass tube plugged with cotton-wool. 

 The whole apparatus is then heated sufficiently to render it 

 sterile, after which the tube is placed in a horizontal position, 

 so that the gelatine sets in a layer in the lower part of the 

 tube. When the air is to be examined, the outer india-rubber 

 cap is removed, and air slowly drawn through the tube. The 

 germs contained in the air settle down on the gelatine, and 

 after the aspiration is concluded the tube is again closed and 

 placed in the incubator, where some of the germs produce 

 visible colonies, which are easily counted. The results show 

 that with a sufficiently slow current of air, the bacteria, which 

 are often floating about in the air in larger or smaller aggrega- 

 tions, frequently clinging to dust-particles, settle sooner than 

 the mould-spores ; so that the gelatine in the front part of the 

 tube generally contains the majority of the bacteria colonies, 

 whilst the mould-spores develop further along. 



HUEPPE, v. SCHLEN, and others, employ liquid gelatine for 

 air analyses, the air being aspirated through the gelatine, after 

 which the latter is poured onto glass-plates. 



