120 BACTERIA. 



which means there is a freer circulation of air allowed to 

 take place through the fermenting mass than when they are 

 plucked from the stalks and pressed and fermented. Of 

 course, it might be objected that the air is useful merely in 

 carrying the yeast cells into contact with the fermentable 

 fluid, but numerous experiments have been carried on to 

 prove that the presence of oxygen is absolutely necessary 

 for the resuscitation of old and spore-bearing yeast-cells. 

 This is in itself a most remarkable circumstance, and a very 

 significant one when it is borne in mind how very different 

 the conditions are under which the later stages of fermenta- 

 tion are best carried on. It must be remembered, however, 

 that the conditions most favourable for the multiplication 

 and production of a sufficient number of active cells are not 

 by any means the conditions most favourable for the decom- 

 position by the cells of the largest amounts of sugar 



In our large breweries (as is sometimes brought home to us 

 only too closely by the reports of the death from suffocation by 

 carbonic acid gas, of men who go down into vats to clean them 

 out) there is always, as the fermentation process goes on, a very 

 great accumulation of carbonic acid gas on the surface of the 

 fermenting liquid ; so dense and so deep is this layer that it is 

 at once seen how impossible it is for much free oxygen from 

 the atmosphere to obtain access to the yeast-cells. Any 

 oxygen they utilize for the building up of their protoplasm 

 must be derived either from air held in solution in the fer- 

 menting liquid (which can only be a very small amount, as 

 the boiling of the wort must have driven out a very large 

 proportion of such air from the fluid), from the small amount 

 of oxygen that can pass through the layer by diffusion, or it 

 must be derived from the breaking down of those substances 

 rich in oxygen that are contained in the malt solution. 



In the same way in the later stages of wine-making the fer- 

 mentation is allowed to go on in large casks ; carbonic acid gas 

 here also rises to the surface, fills the cask up to the bung-hole 

 and gradually flows over, and, as the carbonic acid gas comes 

 in minute bubbles to the surface from all parts of the fluid, it 

 can scarcely be imagined that any large amount of free oxygen 

 can be left in suspension in the fermenting grape juice ; and 

 certainly very little can find its way from without, as the car- 

 bonic acid gas is pouring out from the bung hole in such 

 considerable quantities. The fermentation at this stage, then. 



