240 THE PRESERVATION OF FOOD PRODUCTS. 
upon for the curing of hay, so a different principle is used. If the 
moist grass is heaped in piles and allowed to stand for a few days, 
there appears a marked rise in temperature. This continues rapidly, 
the rate and the temperature reached depending upon the conditions : 
the denser the packing, the higher the temperature. Commonly 
the rise is not above 160, although under some conditions it goes 
above this, even to the point of spontaneous combustion. This latter 
phenomenon is of rare occurrence, however, although probably it 
is the cause of the spontaneous combustion that occurs occasionally 
in a barn when the hay is packed away in too moist a condition. 
The cause of such self -heating has not been definitely settled. 
It is evident that the phenomenon has a decided resemblance 
to the fermentation of tobacco and also to that of silage. 
Three possible causes may be concerned: i. The respiratory 
changes in the still living cells of the grass. 2. The action of 
enzymes from the grass. 3. The growth of microorganisms. 
Experimental tests have not yet settled positively the relation 
between these possibilities. Sterilized hay will not undergo this 
heating, while the same sterilized hay, if inoculated with certain 
species of microorganisms (Outturn), will show a rise in temperature 
apparently identical with the self-heating. This would clearly 
indicate that microorganisms 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 microorganisms alone are concerned in the 
phenomenon. Further, it is extremely improbable that bacteria 
or molds could develop heat sufficient to kill 'themselves, 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 respiratory processes; the enzymes in 
the grass probably also start some chemical action and, lastly, the 
microorganisms on the grass, by their growth, add to the fermenta- 
