INVISIBLE DOMESTIC SERVANTS 55 



consists of innumerable rod-like bacteria, cohering into 

 threads, and imbedded in a structureless jelly. The 

 vinegar-plant is a mass of bacteria, glued together by 

 their own exudation. Besides the bacteria which are 

 entangled in the membrane, scattered rods and still smaller 

 spores are found in the liquid beneath, and these too can 

 produce a new film if placed in a suitable liquid. 



With a little management we can see the growth of a 

 vinegar-plant for ourselves ; we can see the single rods 

 adding length to length until they form threads, and the 

 threads branching to form new threads. They grow like 

 the simplest kinds of plants, and it is not incorrect to call 

 the membrane a vinegar-plant, though it is rather a col- 

 lection of plants than a single one. 



Vinegar-bacteria are much smaller even than the minute 

 yeast-cells. A globular yeast-cell has a diameter of about 

 yj^ mm., but a rod of the vinegar-bacterium is usually 



on ly 5^0 mm - lon g- 



The rod-like cells of the vinegar-plant cannot maintain 

 themselves in full activity without air, and thrive best 

 at the surface of the nutritive liquid. The jelly which 

 they exude is really a contrivance for keeping vast numbers 

 of cells floating in contact with the air. 



When vinegar is formed from sugar by any natural 

 process, alcohol-formation is usually an intermediate step. 

 First the sugar is made to yield alcohol, and then the 

 alcohol is made to yield vinegar. In the manufacture of 

 beer or wine the sugar of the malt or grape is first changed 

 to alcohol by the action of yeast- cells. When the alcohol 

 has once been formed, it may be changed to vinegar by 

 the action of another kind of minute cells, the vinegar- 

 bacteria. This often happens against the will of the 

 brewer or wine-merchant ; his liquor turns sour, and is 

 spoilt thereby. It is the same or a very similar vinegar- 

 plant which does the acidifying of the alcohol both here 

 and in the domestic manufacture of vinegar. Knowledge 

 of the fact that the formation of alcohol is a preliminary 



