54 HOUSE, GARDEN, AND FIELD 



Yeast-cells are very small, only the hundredth of a 

 millimetre in diameter. Suppose a man were to shrink 

 until he had only the height of this letter (I), then a 

 threepenny-piece in his pocket, if reduced in the same 

 proportion, would be rather too big to represent the 

 diameter of a yeast-grain. 



In my household we now and then make a brew of 

 vinegar, using this old-fashioned recipe. We take half a 

 pound of coarse sugar, a quarter of a pound of treacle, 

 half a pint of vinegar, and three pints of water, pour 

 these together into a wide-mouthed stone jar, loosely 

 closed to exclude the dust without excluding the air, and 

 keep it as near as may be at summer heat (28 C., about 

 82 F.) for three months. Then we find in our jar near 

 half a gallon of serviceable vinegar. Some add fresh 

 currants or gooseberries to the mixture before brewing ; 

 others put in a toast dipped in yeast, but these things 

 make no appreciable difference to the result. 



There is one other ingredient which I have not yet 

 mentioned. Old housekeepers often left it out, but it 

 makes the yield of vinegar more certain, and we never omit 

 it. This is a piece of a " vinegar-plant," or " mother of 

 vinegar," as some call it ; the Yorkshire name is " mothers." 

 I will next try to explain what a vinegar-plant is. 



When the three months are up, we find a jelly-like cake 

 floating on the vinegar ; it will probably be about a 

 quarter of an inch thick. This cake begins to form as an 

 extremely thin membrane, and grows rapidly until it has 

 attained something like its ultimate thickness. If dis- 

 turbed, it becomes covered with the liquid, and a new 

 membrane forms above it. Its form is adapted to that 

 of the enclosing vessel. When lifted out by the hand, it 

 is found to be soft, brownish, almost transparent and 

 tough, resisting any attempt to tear it across, but easily 

 splitting into layers. This membrane is the vinegar- 

 plant or mother of vinegar. 



High powers of the microscope show that the membrane 



