128 NUTRITION AND METABOLISM. 



product, hydrogen sulphide, which is oxidized by the sulphur bacteria 

 first to sulphur, then later to sulphates. 



PHOSPHORUS CYCLE. The cycle of phosphorus has not been worked 

 out completely, but from the discussion in the last pages, it is plainly seen 

 that a simple cycle very much like the ones above must exist. It is 

 probably much simpler because phosphorus does not enter as easily into 

 organic compounds as nitrogen. 



PHYSICAL PRODUCTS or METABOLISM. 



PRODUCTION or HEAT. It has long been known that fermentation 

 produces heat. The rise of temperature is usually not very great. In 

 lactic fermentation it amounts to about i, in alcoholic fermentation to 

 2 or 3, but in certain processes the heat liberated is considerable, as in 

 the fermentation of manure, of ensilage, of vinegar, and in others. 



The cause of heat formation is quite evident from the discussion on 

 page 98. Organisms decompose organic matter which means a liber- 

 ation of energy. Part of this energy is used for the continuation of life- 

 processes; the rest, usually the larger part, is not employed and appears 

 in the form of heat. The amount of heat produced can be measured 

 directly with the thermometer if great care is taken that no heat is lost 

 by radiation or by evaporation of water. 



The best known fermentation of this character is the vinegar fermen- 

 tation. In the quick-vinegar process (page 456) the temperature rises 

 sometimes as high as 10 to 15 above the temperature of the room and 

 the vinegar manufacturer uses the heat produced by the bacteria to keep 

 the generators at the optimum temperature. If the process is not con- 

 trolled carefully, the vinegar bacteria are likely to produce sufficient heat 

 to kill themselves. 



The heat produced in the fermentation of manure, especially horse 

 manure, is used in the hot-beds to cultivate and force young plants. In 

 the manure pile, great heat production is not desirable because high 

 temperatures will volatilize the ammonia; the tight packing of manure 

 which keeps out the oxygen will prevent too strong bacterial action. The 

 highest temperature in silos which has been recorded is about 70, but 

 the best silage is secured by keeping the temperature below 50. Ensilage 

 fermentation is not thoroughly understood, however, and no accurate 

 statements can be made as to the cause of the increase in temperature. 



