FIRING OF HAYSTACKS. 247 



soil can work energetically at 70 or higher. Eventually, how- 

 ever, even they will be killed by the high temperature, and 

 direct oxidation of some substances of the hay must then 

 commence. 



A further exaltation of temperature then ensues until, under 

 favourable conditions, the actual ignition point of the hay is 

 reached and the mass begins to inflame. Generally, however, 

 . the highest temperature is reached near the centre of the stack, 

 where access of air is difficult and slow ; consequently a 

 smouldering combustion rather than actual flaming results. 



The conditions most favourable to the dangerous heat- 

 ing of haystacks are not exactly known, but undoubtedly the 

 presence of a certain amount of moisture is one of the most 

 important. Obviously, then, one of the best methods of pre- 

 venting the firing of haystacks is to thoroughly dry the hay 

 before stacking it. This cannot always be done, nor is it 

 entirely satisfactory, for if over-dried it does not ferment suffi- 

 ciently to develop the aroma, flavour, and colour which are 

 desirable in good hay. If hay has to be stacked while some- 

 what green, one of two methods is generally adopted : 



(1) To mix salt with it. This probably acts by preventing or 



hindering the growth of the micro-organisms. At the 

 same time it renders the hay more palatable and also 

 probably lessens the risk of its becoming mouldy. 



(2) To ventilate the stack. This is probably the more efficient 



method. It is generally effected by including in the 

 hay at the time of stacking a sack or sacks filled with 

 hay, placed vertically and drawn upwards from time to 

 time as the stack rises. Another method, often em- 

 ployed in Scotland, is to erect, in the centre of the site 

 chosen for the stack, an open conical structure com- 

 posed of wooden scantlings meeting at a point. The 

 stack is then built round and above this. These 

 methods depend for their efficiency not upon checking 

 the heat production, as the first one does, but upon the 

 removal of the evolved heat by circulation of air. 

 The odour of hay, upon which its palatability largely de- 

 pends, is probably due partly to the products of fermentation, 

 among which compound ethers are probably present, and 



