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found the vegetation thereby promoted. * And by the experi-ments of Priestley, Saussure, See. it appears that a moderatemixture of carbonic acid gas in the atmosphere of growing plantsgreatly promotes vegetation. But, in fact, the learned Lord doesnot mean, by the above general position, that the fermentationof animal and vegetable substances, preparing for manure, shouldbe violently excited, or continued till the substances are decomposedand reduced to their first principles, as some doctors in agriculturehave taught. In his letter to the President of the Board of Agri-culture, he explicitly declares that moderate compression of thiscomposite substance, as it excludes the too free ingress of exter-nal air, is proper ; that it should be applied to the soil when it isbut imperfectly fermented, and while the process of fermentationis going on; and to this purpose, he adduces the evidence ofgardeners, who know that dung which has been used in hot-beds is of little value compared with fresh dung.f As the gaseswhich are disengaged and dissipated by a rapid fermentation arethe real ingredients of vegetables,—as all the aliment which hu-man industry can bestow to promote the growth of plants, mustbe conveyed in solution to the roots, it would seem extremelypreposterous and unfrugal to expel any of these ingredients inthe air, which might be detained in solution.
But those who reduce manure to the last stage of putrefactionbefore they use it, seem to think those volatilizable ingredientsmust necessarily fly off, whatever be the mode of preparation ; fand the extreme specific levity of hydrogen makes that supposi-tion more plausible, with respect to that gas. But notwithstand-ing the great specific levity of hydrogen, which impels it tomount to the etherial regions, it has other propensities, whichfrequently detain it long below. By the force of affinity, itcleaves to phosphorus in marshes, rotten wood, fish, &c.; it cleavesmore strongly to carbon in the composition of oil, a combinationso durable that it is not relaxed by the process of the soap ma-nufacture, by which it is made soluble in water, and then provesthe nourishment of growing plants. The attachment of hydro-gen to carbon, in the juice of farm dung macerated in water,though not so strong, is somewhat similar; as is evinced by theoily appearance and clammy feel of this liquid, and the strongsmell of wet hydrogen which it emits when stirred. Nor is thelavish waste of carbon, expelled in carbonic acid gas, more ne-cessary, in the preparation of manure. The absorption of oxygenmto the mass is less, in proportion as the water contained in it ismore abundant; and consequently, when there is plenty of water,
* Annals of Agriculture, Exper. 19. & 37 . f Communications, p. 384.
f All smelling substances emit their peculiar aroma, by which the olfactoryergan distinguishes one substance from another ; but the bulk or value does notywm to be sensibly wasted by such means. When a wasteful emission occurs, ft
the violence of fermentation which is the cause of it