10 On the Theory of Manures. 



nitrogen is got wholly from the soil, crops should impoverish the 

 soil according to the nitrogen they extract. Many plants which 

 are found to abound in fungi at the roots must excrete a great 

 deal of nitrogen by the process of exosmose, fungi being well 

 known to be voracious of nitrogen for their food. Horse-dung 

 contains about the double of the nitrogen that cow-dung does, 

 yet most crops and most soils are more benefited by the latter 

 than the former. Some crops, as wheat, are found to have 

 gluten deposited in greater abundance in the ear, when manures 

 have been furnished abounding in nitrogen ; the proportion of 

 gluten to the starch being much greater. But when manures 

 containing much nitrogen are applied to oats and barley, the 

 gluten is not increased there as in the wheat. All these cir- 

 cumstances seem to point out that there is not always a regular 

 ratio between the nitrogen deposited in the manures, and that 

 carried oiF in the crops ; and that nitrogen is got in some way 

 not yet understood. It is probable, though not hitherto ad- 

 mitted, that, as plants contain a system of air vessels by which 

 the air containing the greater part of its volume in nitrogen is 

 constantly brought into contact with the circulating juices of the 

 plant, ammonia may be formed from the newly liberated or 

 nascent hydrogen, developed in the transformations of the cir- 

 culating sap, coming in contact with the nitrogen of the air, 

 perhaps also recently deprived of its oxygen by absorption, which 

 is well known to take place. The carburetted hydrogen of the 

 air, stated by Dumas as equal in quantity to the carbonic acid, 

 and the sulj)huretted hydrogen lately found so beneficial by Mr. 

 Solly, may, perhaps, also furnish hydrogen to the plant to assist 

 in the formation of ammonia. The nascent nitrogen from the 

 air, deprived of its oxygen, and confined in the vessels of the 

 plant, if it come in contact with nascent hydrogen there, should 

 be as capable of forming ammonia in that situation as in the 

 manure heap. That plants do get nitrogen, under a form capable 

 of assimilation, in this or some such manner, is evident from 

 their producing it in greater quantity than the ammonia from 

 manure, or that from the air in rain-water to the roots, could be 

 capable of furnishing. It has been generally said by our most 

 scientific writers, that the ammonia is wholly got by the roots : 

 the experiment of Mr. Milne, however, lately narrated in the 

 Gardener's Chronicle, in which, having hung up tin cans contain- 

 ing ammoniacal liquor, and sprinkling it on the floor of a vinery, 

 he found, in 48 hours, the leaves to assume a dark green ap- 

 pearance, and the after-growth to be exceedingly luxuriant, is, 

 I should think, sufficient proof that leaves absorb ammonia 

 from the air when they fall in with it. That nitrogen is not 

 wholly from that provided in the form of ammonia to the roots 

 is evident ; that some plants get it from the air, and that pro- 



