142 THE BOOK OF NATURE STUDY 



without the microscope, but the naked-eye appearances are 

 usually characteristic of each kind and are easily distinguished. 



You have probably noticed that a pot of jam sometimes has 

 an alcoholic smell when opened, the jam itself having a frothy 

 appearance. The jam has " fermented/' All kinds of fermenta- 

 tion in sugary substances are due to the action of minute plants 

 called yeasts. The most familiar kind of yeast is beer yeast 

 or brewer's yeast, used in brewing and in bread-baking. In 

 brewing, the grain (usually barley) is allowed to germinate until 

 the starch (stored in the endosperm) is largely converted into 

 sugar, then the sprouting barley is killed by heat, and the sugar 

 extracted by water and yeast added to the extract. 



If some yeast is rubbed up in water to form a paste, and a 

 little of this is added to a solution of sugar, the liquid soon be- 

 comes turbid, bubbles of gas (carbon dioxide) rise to the surface 

 and form a froth, and the liquid loses its sweetness and smells 

 of alcohol. The turbid sugar solution is found to contain an 

 enormous number of yeast cells, some floating singly, while 

 others are connected in chains. Test the gas given off by sugar 

 solution in which yeast is growing by lowering a lighted match 

 or taper, or a glass rod with a drop of lime-water on its end, into 

 a jar ; or place some yeast containing sugar solution in a flask 

 fitted with a cork and bent tube dipping into lime-water. The 

 alcohol produced may be recognised by smelling and tasting 

 the fermenting sugar solution from time to time, or by warming 

 the liquid in a test-tube or flask fitted with a straight piece of 

 glass-tubing until the alcohol vapour given off can be ignited 

 with a match. Yeast, like bacteria, produces spores, especially 

 when the food supply runs short ; these spores remain alive 

 for a long time, even when dried up, are blown about in the air, 

 and develop into active yeast cells as soon as they reach a suit- 

 able medium as is shown by simply exposing a sugar solution 

 to the air. 



In baking, yeast is used simply for the sake of the gas (carbon 

 dioxide) given off, which forms bubbles and makes the bread 

 light ; the alcohol produced is, of course, driven off by the heat 

 in baking. 



Bacteria are minute plants, whose structure is extremely 



