TEXT-BOOK OF BACTERIOLOGY. 349 



of the large tube is closed by two stiff rubber caps, the inner 

 having a central round opening, the outer being solid. The whole 

 apparatus is then sterilized, for about an hour, in the steam-ster- 

 ilizer. Then remove the rubber plug, pour in 50 c.cm. of sterile, 

 liquid nutrient gelatin, close again, and distribute the mass on the 

 walls of the tube, as in Esmarch's procedure. Put this under 

 a stream of water and roll it on its horizontal axis as quickly 

 as possible. When the gelatin begins to become viscid, the rota- 

 tion should be stopped; the greater mass of the medium will then 

 slowly sink to the most dependent part of the tube, and form a 

 rather thick layer. 



We must see that the tube remains constantly turned down- 

 ward. Fasten the apparatus upon a movable tripod and begin in- 

 vestigation. Connect the small tube in the rubber plug with an 

 aspirator, remove the outer, un perforated cap from the other 

 aperture and let the suction begin. The water flowing over into 

 the lower aspiration-flask is replaced in the upper by inflowing air. 

 But this air, in order to reach the flask, must first pass through 

 the long pipe and its germs will thus find an opportunity of de- 

 positing. 



The result has shown that this happens satisfactorily in the 

 anterior sections of the tube. Even if some micro-organism should 

 be carried off by the current of air to the other end of the tube, it 

 is compelled to settle at the cotton wadding closing the small 

 inner tube. The wadding being likewise soaked with gelatin, these 

 stray germs can develop. 



The velocity of the current of air must, of course, not exceed a 

 certain point. It is determined by regulating the quantity of the 

 water passing between the two aspirator-flasks by cocks, glass 

 tubes, etc. 



One litre of water is generally allowed to run over in about 

 two minutes, just as much air meanwhile passing through the 

 tube. The upper flask having become empty, the vessels are 

 changed by merely shifting their places. The apparatus works 

 quite satisfactorily and has only a single, though very essential, 

 drawback. 



It is impossible to examine large quantities of air as to their 

 bacterial contents in a short time. We dispose, at best, of frag- 

 mentary quantities only, supplied by small parts of the surround- 

 ing atmosphere, but incapable of generalization. 



This defect is remedied by Petri's method, which satisfies all 

 demands and can be designated as perfect. It requires rather con- 

 siderable preparations and a larger amount of special apparatus. 



