66 MICRO-ORGANISMS AND FERMENTATION. 



A true knowledge of these germs can only be obtained when 

 they have developed ; in cases where several germs penetrate 

 into the same flask, the strongest germ would by its growth, in 

 all probability, prevent the development of the others, so that 

 these would not be detected in a subsequent examination. At 

 the same time this method necessitates the opening of a large 

 number of flasks, which makes the operation cumbersome and 

 costly. As the flasks only show what was present in the air 

 at the moment of opening, Erlenmeyer flasks were also used to 

 give supplementary information, for which purpose they were 

 allowed to remain in the same locality for a long time, in some 

 cases as long as 48 hours. 



After these preliminary remarks, we will give a brief 

 summary of the results obtained by Hansen. 



He confirmed the statement made by Pasteur and Miquel 

 that the air at adjacent places, and at the same time, may eon- 

 tain different numbers and different species of organisms ; and 

 he found that this holds good for adjacent parts of one garden. 

 Hansen mentions, amongst other factors determining the 

 distribution of micro-organisms, that those forms, for instance, 

 which in the first half of July commonly occurred under the 

 cherry trees in the garden, were in the latter half of the same 

 month entirely absent from this locality ; further, that organ- 

 isms which at one time could be found under the cherry trees, 

 but not under the vines, were to be found later only under 

 the latter. As a proof of the inequality of distribution of the 

 organisms, he showed that flasks opened in the same place in 

 the same series of experiments often had the most diverse 

 contents. 



The experiments with vacuum flasks have further taught 

 us that the micro-organisms of the air often occur in groups 

 or clouds, with intermediate spaces, which are either germ-free 

 or only contain a few isolated germs. As the organisms are 

 not generated in the air, but on the earth and on fruit, it 

 follows that their presence in the air must be dependent on 

 the condition of the surface of the ground and of the fruit, 

 which again depends to some extent on the weather. 



Hansen's numerous analyses have further proved that the 

 Saccharomycetes comparatively seldom occur in the dust of 



