3l6 AGRICULTURAL BACTERIOLOGY. 



away in the barn it "heats" in the mow almost universally, 

 sometimes indeed to such a high temperature that there is 

 danger of ignition. This is evidently due to fermentative 

 changes. The hay changes its character while lying in the 

 mow, and it is clear that the change is closely akin to that 

 which occurs in the so-called brown hay. 



The whole subject of the curing of hay needs further study 

 before we can understand its real nature or the importance of 

 these fermentation processes. Until these facts are better 

 known we cannot determine the best method at the disposal 

 of the farmer for curing and preserving this very important 

 food product. 



Certain phenomena sometimes seen in cotton are clearly 

 closely akin to the fermentation just described. Sometimes 

 cotton undergoes a spontaneous heating sufficient to render it 

 in danger of combustion, and this must be due to processes 

 similar to those just described. The same thing is true of 

 hops which occasionally develop a similar spontaneous heating 

 during the curing. 



USE OF COLD. 



The use of low temperature is the oldest method of prevent- 

 ing bacterial action. All the common species of bacteria 

 grow more slowly as the temperature is lowered, and all 

 cease growing entirely when it reaches freezing. The nearer 

 to freezing a fermentable substance is kept, the greater the 

 delay of the bacterial growth. In the large cold-storage 

 houses the food which is to be preserved is cooled to a tem- 

 perature below freezing and is, consequently, actually frozen. 

 At this temperature the bacteria never act and the material 

 may be kept indefinitely. It must be remembered, however, 

 that the low temperatures do not kill the bacteria but only 

 delay their action, and as soon as such food products are 

 warmed the bacteria begin their action immediately. 



