EXAMINATION OF AIR AND WATER. 43 



which were boiled for a certain time, or of flasks of the same 

 kind as Pasteur's vacuum flasks, the necks of which were 

 drawn out to a fine point, and were closed with sealing-wax 

 whilst boiling. A little below the point a notch was made 

 with a file, in order that the point might be easily broken off 

 when it was desired to admit the air. 



When these flasks had become filled with the air of the 

 locality to be examined, they were again closed with sealing- 

 wax and thoroughly shaken in order to mix the contents of 

 the infiltrated air with the liquid The flasks were then put 

 aside for a shorter or longer time, up to six weeks, and their 

 contents examined under the microscope. 



In these investigations Hansen often found that the wort 

 remained bright and apparently unchanged, even although 

 a growth had taken place. Hence, the examination with 

 the naked eye alone cannot be relied on. He names the 

 following forms which, when present in a feeble state of 

 growth, cannot be detected macroscopically : Aspergillus, 

 Mucor, Penicillium, Cladosporium, Bacterium aceti and 

 Pasteurianum, and Mycoderma cerevisise. Even when these 

 micro-organisms have formed vigorous growths, the above- 

 mentioned nutritive liquid has remained bright. 



It was further shown that pure cultures may often be 

 obtained by the use of these flasks, when only one species was 

 drawn into the flask with the air. It very seldom happened 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, however, 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 



