AIR AND WATER. 65 



be very difficult to separate the individuals from each other 

 by agitation. 



E. C. Hansen's investigations of the air were made between 

 1878 and 1882. His main object was to throw light on ques- 

 tions affecting the fermentation industries. As is well 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 used 

 consisted either of Erlenmeyer 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 Pasteur ianum, and Myco- 

 derma cerevisice. .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 

 gained access to the flask along with the air. It very seldom 

 happened that three or four species were found in the same 

 vessel. This arises from the fact that only a very small volume 

 of air enters each flask. The advantages of this are evident : 



