PRESERVATION OF DRYING 250 
apparently identical with the self-heating. This would clearly 
indicate that microdrganisms may be prominently concerned 
in the process. But the sterilizing kills the plant cells so that 
the respiratory changes are stopped, and also destroys most of 
the enzymes present. Hence the fact that sterilized hay will 
not thus heat is no proof that microdrganisms alone are con- 
cerned in the phenomenon. Further, it is extremely improbable 
that bacteria or molds could develop heat sufficient to kill them- 
selves, still less sufficient to cause a spontaneous combustion. 
No experiment with organisms has given a heat higher than 160° 
as the result of their action. Hence, the extreme heat must be 
due to other causes. Probably this fermentation, like many 
another, is of a mixed nature. The moist grass still contains 
some living cells that for a time remain alive and carry on respira- 
tory processes; the enzymes in the grass probably also start 
some chemical action and, lastly, the microérganisms on the 
grass, by their growth, add to the fermentative changes going on 
in the hay. Asa result of all three, the temperature rises. 
This self-heating is utilized in some countries to cure the hay. 
The grass is built up into a stack or rick 13 to 16 feet high, and 
16 to 24 feet in diameter. It is well trodden down, but not firmly 
packed, and the whole stack is thatched so as to shed the rain. 
In such ricks a spontaneous fermentation sets up and the mass 
becomes heated. The temperature frequently rises as high as 
160°F., but not much higher, and there is no danger of spontaneous 
combustion. The rick is not opened, but the hay remains in the 
mass until the farmer wishes to use it. It is immaterial whether 
the hay is rained on or not, and this makes the process especially 
adapted to rainy districts. 
The fermentation which takes place in these ricks produces a 
great change in the nature of the product. It becomes a firm, 
dry mass, of a pale or dark brown color or, if the heating is too 
great, it may be almost black. It has developed at the same time 
an aromatic odor which resembles freshly baked bread. There 
