PLANTS WHICH CAUSE DECAY 387 



water by putting ice around the vessel containing it 

 rather than in it. 



In the case of eggs, bacteria often gain entrance 

 before they are laid ; if this is not the case they can be 

 preserved by keeping the bacteria out. For this purpose 

 the pores of the shell are filled by dipping them into 

 water-glass (vaseline and other substances have been 

 used; they are also packed in brine, etc). 



Prominent among the bacteria of fermentation are, 

 in addition to the lactic -acid bacteria of milk, the 

 vinegar -making bacteria. In order to study these bac- 

 teria it is only necessary to take a little "mother" 

 (which is a gelatinous mass containing the bacteria) 

 from vinegar and place it in a little hard cider or a 

 weak solution of alcohol (containing not more than 6 

 or 7 per cent alcohol) neutralize with lime-water, add 

 enough liquid litmus to give a good blue color and allow 

 it to stand in a warm place (light should be excluded) . 

 If the liquid be poured over a mass of excelsior or 

 shavings, the vinegar is formed with great rapidity, as 

 shown by the color of the litmus. 1 The reason is that 

 in this case the bacteria are abundantly supplied with 

 air, which is essential to their activity. This experi- 

 ment may be conveniently carried out in a covered 

 wooden pail or tub filled with excelsior (or shavings) . 



In recent years the use of silage has become very 

 extensive. Silage is made by filling a pit or other 



1 Ordinary litmus paper may be used in place of liquid litmus. 



