NATUKE AND PROPERTIES. 255 



high temperature ; whereas ammonia, consisting of nitro- 

 gen and hydrogen, is formed when those substances 

 decompose in the absence, or nearly so, of atmospheric 

 air. Organic substances decomposing under the latter 

 condition emit an objectionable pungent odour, which 

 must partly be attributed to the formation of ammonia. 

 Tobacco, soon after harvesting, commences, according to 

 the conditions under which it is placed, one of these 

 decompositions. The extent of the decomposition the 

 tobacco has gone through may be partly judged from the 

 colour the leaves have attained. If leaves be dried so 

 rapidly as to remain green, the decomposition is probably 

 confined to the formation of carbonic acid. A yellow 

 colour indicates the formation of nitric acid ; and a dark- 

 brown or black colour, that of ammonia. The conditions 

 under which nitric acid and ammonia are formed being 

 known, it is possible to control their development. When 

 the tobacco is hung far apart, so that the air has free 

 access, the formation of nitric acid will take place ; but if 

 the air be excluded more or less, by hanging the tobacco 

 very close, or pressing it in heaps or pits, the formation 

 of ammonia is engendered. 



Nitric acid generally promotes the combustion of plant 

 substances, by supplying a portion of the needed oxygen, 

 and has undoubtedly a similar effect in tobacco ; its oc- 

 currence in the tobacco is therefore a desideratum with 

 the cultivator and manufacturer, and to supply any 

 deficiency, the manufacturer often resorts to impreg- 

 nating his tobacco with a solution of saltpetre. From 

 this, however, it must not be concluded that every 

 tobacco containing a large amount of nitric acid will 



