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 (Oidium), 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. 

 T^he 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- 



