140 Retrospective Criticism. 



and, as he himself says, the crop planted with fresh unrotted dung loses the 

 benefit, and much is lost before another crop succeeds. The chemical pre- 

 paration, or digestion, of the manure intended as the food of plants is, un- 

 doubtedly, best done in the rot-heap. If carefully managed, covered with mould, 

 and kept as directed in the last essay, there should not be so much loss as the 

 professor states, one half of the nitrogen. It is not convenient at all times to 

 deposit fresh manure, and the carriage is much more expensive. If the heat 

 is well kept down by frequent turning, and the washing away prevented, there 

 should not be much loss. Putrefaction and fermentation are much more 

 active in the heap than in detached portions. Fresh manure in the potato- 

 drill, unless the season is moist, is often found quite fresh at the end of the 

 year. 



On the subject of nitrogen he notices only that got from the nitrogen of the 

 manure in the state of ammonia; but this, though undoubtedly the principal, 

 is not the only source of nitrogen. If, as asserted by Dumas and others, 

 animals do not absorb nitrogen from the air, their nitrogen being wholly got 

 from plants, it follows there must be some source of supplying the waste. 

 Accordingly we find that, in the combustion of coal and wood, the hydrogen 

 given oif forms ammonia with the nitrogen of the air already deprived of its 

 oxygen by combustion ; and part is found deposited in the soot in the form of 

 carbonate of ammonia, or sulphate, when the substances burned contain 

 sulphur ; part of the ammonia will also escape into the air : and thus com- 

 bustion is a great source of nitrogen to plants, as in soot, gas liquor, &c., be- 

 sides that to the soil by rain. Volcanoes are also a source of ammonia on 

 a large scale, as noticed by Professor Daubeny. The eremacausis of Liebig 

 (or slow combustion of substances) is also another source. Where the oxygen 

 is partly got from water and partly from the air, the hydrogen set free in the 

 one case, and the nitrogen in the other, will form ammonia. Professor John- 

 son seems to think that much of the ammonia said to be absorbed by charcoal, 

 &c., should be ascribed to this source rather than to absorption. In the manure 

 heap a good deal of the ammonia found is probably due to this source, as well 

 as that of the nitrogenous substances it contains. Nitrogen is also soluble 

 in small quantity in water; and the water of the soil absorbed by plants will, no 

 doubt, afford a small portion of nitrogen. The common air absorbed by 

 plants, and deprived of its oxygen by absorption, is another source, as noticed 

 in our last essay. From all these sources the nitrogen is supplied to plants 

 in sufficient abundance to enable them, on the other hand, to supply the 

 wants of animals, which are now generally believed to be consumers rather 

 than producers of nitrogen: they give it off principally by the urine, show- 

 ing the great benefit of retaining this in the manure heap ; but also waste 

 it by perspiration and exhalation, as shown by the fetid smell of both these 

 excretions. 



Dr. Madden is of the same opinion as the professor, that sulphate of am- 

 monia is very apt to be re-acted on again by carbonate of lime ; and sulphate 

 of lime and carbonate of ammonia are the result. If soils contain much 

 lime or chalk, the benefits of urate or sulphate of ammonia may be greatly lost 

 by this cause. Sulphuric acid is said to be as cheap in proportion as gypsum, 

 where needed ; but carbonate and humate of ammonia we should consider 

 more beneficial to the generality of plants, though such as clover, pulse, &c., 

 are more in need of sulphur. — R. L. Kilmarnock, Dec. 30. 184.2. 



Charcoal and Charcoal Dust. — In your Vol. for 1841, p. 254-5., it is said, 

 speaking of charcoal and charcoal dust, that M. Lucas was the first to show 

 the action exercised by the charcoal on vegetation ; thus setting aside the 

 Italians, among whom the Abbe Piccone and Professor Moretti have treated 

 of it at length in vol. 2. of the Biblioteca As,raria, p. 70. — Giuseppe Manetti. 

 Monza, Dec. 7. 1842. 



