HEAT DEVELOPMENT DUE TO BACTERIAL ACTIVITY 133 



Of still greater economic importance is a knowledge of the conditions 

 which prevent the spontaneous heating of stored corn and other grain. 

 The damping of such substances, carried as cargoes, has caused many a 

 ship to catch iire. The flora on the surface of stored corn has been 

 investigated. Bacteria, yeasts and moulds are found in great num- 

 bers ; there may be as many as eleven millions per gram. Of course 

 the presence of most of these does not affect the corn one way or the 

 other, but amongst them are some organisms which can take advantage 

 of the corn when in a damp condition and effect multiplication at its 

 expense. A rise in temperature is the result. The amount of moisture 

 present in the corn determines which of the different kinds of 

 organisms obtain the upper hand. If the percentage is between 15 

 and 25, certain spore-forming bacteria, and if as much as 30 one or 

 more of the moulds, predominate : in either case, a rise in temperature 

 is the result. It is obvious that the best method of protecting stored 

 grain from the ravages of these organisms is to store it in as dry a 

 condition as possible and then protect it from rain and from absorption 

 of moisture from the atmosphere. 



The modes of preparation of burnt hay and of tobacco are described 

 in a later chapter. It is sufficient to note here that the mown grass in 

 the one case, and the tobacco leaves in the other, are massed together 

 in a humid condition in order to promote the desired development of 

 heat. 



As is well known, the ultimate result of allowing materials like 

 damp hay or damp cotton-waste to remain massed together is spon- 

 taneous combustion. The temperature of the humid mass rises to the 

 ignition-point, and bursts into flame. Microorganisms are not, how- 

 ever, able to raise the temperature to the point of ignition, as this is a 

 good deal higher than the thermal death-point of living organisms. 

 Hence other agencies must in addition be invoked to explain the rise of 

 temperature from about 70° C, which is the highest temperature at which 

 bacteria are known to be able to thrive, up to the temperature of 

 ignition. These agencies are very imperfectly understood. It is pro- 

 bable that the most important of these is the activity of the oxydases or 

 oxidising ferrn,ents that are present in the humid vegetable mass. These 

 bring about certain chemical changes which are attended by the 

 liberation of a considerable amount of heat. 



We must distinguish between the thermogenic or heat-generating 

 and the thermophilic or heat-loving bacteria. As already stated, all 

 bacteria generate a certain amount of heat, but those only are called 

 thermogenic which generate so much heat that the temperature of the 



