EXAMINATION OF AIR AND WATER. 43 



that bacteria, as shown by PETRI, often occur in aggregates in 

 the air, and these will either fall directly onto the gelatine- 

 plate or become mixed in the liquid gelatine, where it will be 

 very difficult to separate the individuals from each other by 

 agitation. 



HANSEN'S investigations of the air were made from 1878 

 to 1882. The main object of his investigations was to throw 

 light on questions affecting the fermentation industries. As is 

 known, his researches on Saccharomyces apiculatus (1880) 

 were partly based on work of this nature. Since the question 

 concerned the organisms which occur in brewing operations, 

 the choice of a nutritive liquid was easily determined, namely, 

 wort as ordinarily employed in breweries. The apparatus 

 employed consisted either of ordinary boiling flasks closed with 

 several layers of sterilised filter-paper, the contents of which 

 were boiled for a certain time, or of vessels similar to Pasteur's 

 vacuum flasks, the necks of which were drawn out to a fine 

 point, and closed with sealing-wax while the contents were 

 boiling. A little below the point a scratch was made with a 

 file, so that the point might be easily broken off when it was 

 desired to admit air. 



When these flasks had been 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 longer or shorter time, lasting in some cases for 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 Myco- 

 derma cerevisise. Even when these micro-organisms have 

 formed vigorous growths, the wort used 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 



