BACTERIA IN NATURAL PROCESSES. 113 



silo is a large, heavily built box, which is open 

 only at the top. In the silo the green food is 

 packed tightly, and when full all access of air is 

 excluded, except at its surface. Under these 

 conditions the food remains moist, but neverthe- 

 less does not undergo its ordinary fermentations 

 and putrefactions, and may be preserved for 

 months without being ruined. The food in such 

 a silo may be taken out months after it is packed, 

 and will still be found to be in good condition foi 

 food. It is true that it has changed its charac* 

 ter somewhat, but it is not decayed, and is eagerly 

 eaten by cattle. 



We are yet very ignorant of the nature of the 

 changes which occur in the food while in the silo. 

 The food is not preserved from fermentation. 

 When the silo is packed slowly, a very decided 

 fermentation occurs by which the mass is raised 

 to a high temperature (140 F. to 160 F.). 

 This heating is produced by certain species of 

 bacteria which grow readily even at this high 

 temperature. The fermentation uses up the air 

 in the silo to a certain extent and produces a 

 settling of the material which still further ex- 

 cludes air. The first fermentation soon ceases, 

 and afterward only slow changes occur. Certain 

 acid-producing bacteria after a little begin to 

 grow slowly, and in time the silage is rendered 

 somewhat sour by the production of acetic acid. 

 But the exclusion of air, the close packing, and 

 the small amount of moisture appear to prevent 

 the growth of the common putrefactive bacteria, 

 and the silage remains good for a long time. In 

 other methods of filling the silo, the food is very 

 quickly packed and densely crowded together so 

 as to exclude as much air as possible from the 



