40 VETEEINAEY HYGIENE 



Manure when placed in a pit undergoes decomposition 

 and fermentation ; it is during the latter process that the 

 needful conversion into ' rotten ' manure occurs which the 

 farmer aims at producing. The formation of this is greatly 

 accelerated by the presence of urine and fsces, in fact the 

 straw by itself would be of very slight manurial value. As 

 fermentation proceeds the temperature of the mass rises, 

 and with it is given off ammonia which is probably the 

 most valuable constituent of the manure. 



The fermenting mass gives out free ammonia and car- 

 bonic acid to the air, and these may be detected even 

 several yards from the pit. As the result of observations 

 it was found that a pit should not be nearer to a stable 

 than thirteen yards. 



The air of marshes is impure from the presence of 

 decomposing vegetable matter. Carbonic acid, marsh 

 gas, and sulphuretted hydrogen are present, the latter 

 sometimes in considerable quantity. The organic matter 

 consists of various bacteria, fungi, algse, and vegetable 

 debris. 



Impurities by Resfiration. — Air is rendered impure by 

 the respiration of men and animals, its carbonic acid and 

 watery vapour are increased, its oxygen diminished, and a 

 proportion of organic matter added to it. So long as 

 respiration is occurring in the open air these impurities 

 are got rid of as fast as they are formed, but in the air of 

 buildings it is different, here they accumulate unless means 

 are at hand for getting rid of them. The employment of 

 such means is known as ventilation ; ventilation and im- 

 purities by respiration are intimately mixed up, a proper 

 understanding of the one is essential to the working of the 

 other. 



At each respiration — the number of which in repose 

 varies with different animals — the air passing to the lungs 

 contains 20*96 per cent, by volume of oxygen, and '06 per 

 cent, of carbonic acid. On its return from the lungs it is 

 found to contain oxygen 16-96, carbonic acid 4-04, while 

 the nitrogen in each case is unaffected. In other words 



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