176 THE WORLD'S LUMBER ROOM. 



The spores of the fungi, which answer to the seeds of 

 other plants, are microscopic, even the largest of them, 

 and the smallest are hardly visible when magnified 360 

 times. Their numbers are so enormous that one single 

 plant may produce multitudes such as the mind cannot 

 realise, and being so extremely minute, clouds of them are 

 constantly suspended in the air, ready to settle and take root 

 on any suitable soil, such as jam, paste, cheese, or even an 

 old boot if left in a damp place. 



As long as this blue-green "mould " grows on the surface 

 of a mass of paste, it can obtain from the air the necessary 

 oxygen ; but if buried in it it does not die, for then it de- 

 composes the starch of the flour, takes its oxygen, and gives 

 off bubbles of carbonic acid. The "bubbling" which 

 we call fermentation, is, according to Professor Tyndall, 

 just " life without air." 



The yeast-plant, another of these ferment-producing 

 fungi, and the only one which can be said to be "cultivated," 

 produces hardly any fermentation if allowed to grow on the 

 surface, where it gets oxygen from the air, but if buried in 

 the wort, it has to decompose the sugar in order to obtain 

 it, and fermentation proceeds so rapidly that streams of 

 carbonic acid flow over the sides of the vat. A drop of 

 yeast, the size of a pin's head, increases enough to ferment a 

 pint of liquid, alcohol, as well as the carbonic acid, and a 

 small quantity of glycerine, being formed in the process. 



Yeast mixed with dough has a similar effect. Part of 

 the starch is transformed first into sugar, then into carbonic 

 gas and water, and the gas in its efforts to escape makes the 

 minute bubbles which render the bread light and spongy. 



