60 MICRO-ORGANISMS AND FERMENTATION. 



The majority of air analyses have been undertaken with 

 a view of throwing light on the obscurity which surrounds 

 most contagious diseases, nearly all of which are, as is 

 well known, attributable to the agency of micro-organisms. 

 With regard to the organisms of fermentation, these have been 

 investigated by Pasteur, and, later, especially by Hansen. The 

 French savant stated that, whilst these germs are always 

 floating about in the air, they are present in much larger quan- 

 tities in the dust which settles on the vessels and apparatus employed. 

 The actual fungi giving rise to alcoholic fermentation are 

 present in comparatively small numbers in the air, whilst the 

 germs of moulds are more frequent ; he further showed, 

 as was subsequently done by Tyndall, that the germ-contents 

 of the air vary both with regard to quantity and species. 

 These results were obtained by exposing beer-wort, wine- 

 must, or yeast-water containing sugar, in open, shallow dishes, 

 at different places, and examining their contents after some 

 time for microscopical germs. Pasteur also employed for 

 this purpose the so-called vacuum flasks, containing nutritive 

 liquids and rarefied air. On opening the flask a sample of 

 germ-laden air could be drawn in. 



The most important air analyses undertaken in recent 

 years are, without doubt, those carried out by Miquel, the 

 director of the laboratory specially arranged for this purpose 

 at Montsouris, near Paris. His fellow-worker, Freudenreich, 

 has also made valuable contributions to our knowledge of this 

 subject. 



Miquel performed his first experiments with a so-called 

 Aeroscope (Fig. 14), which is constructed in the following 

 manner : A bell-shaped vessel, A, is provided with a tube, C, 

 through which air can be aspirated. A hollow cone, shown in 

 the left-hand figure, is screwed into the bottom of A ; the mouth 

 of the cone, B, points downwards ; in the apex, D, of this cone 

 there is a very fine opening through which the air is aspirated, 

 and immediately over this opening is fixed a thin glass plate 

 covered with a mixture of glycerine and glucose. The particles 

 carried in by the air settle to some extent on the viscous mix- 

 ture. The intercepted micro-organisms are distributed as 

 equally as possible on the glass plate, and counted under the 



