44 MICRO-ORGANISMS AND FERMENTATION. 



that three or four species were found in the same flask. This 

 arises from the fact that only a very small volume of air 

 enters each flask. The advantages of this are evident : 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 some length of 

 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 first made by PASTEUR, that 

 the air at adjacent places, and at the same time, may contain 

 different numbers and different varieties of organisms ; and he 

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

 HANSEN states, in discussing the distribution of micro-organ- 

 isms, 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 

 their locality ; further, that organisms which at one time were 

 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 shows that 

 flasks opened in the same place in the same series of experi- 

 ments 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, it follows that their 

 presence in the air must be dependent on the condition of the 

 surface of the ground, which again depends, in certain respects, 

 on the weather. 



