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such as contain tan and gallic acid in a concentrated state, arepernicious to vegetation, when they begin to be decomposed, ifthey are not laid so thick over growing plants as to smother them.On the contrary, wherever straw lies thinly scattered, a superiorverdure arises, in a short time. The animal substances of thecomposition are somewhat different from one another, and differmuch from entire vegetables. The dung of domestic animals,as has been already said, is partly animal matter, and partly ve-getable, on which the first changes have commenced ; but in itsmost recent state, it is not pernicious to vegetation. When theparts of horses’ dung are a little separated and scattered on grassground, the herbage is not injured, and after the first shower, afine verdure appears. The dung of sheep, when it is not accom-panied with urine (which indeed suspends vegetation where itfalls), has the same effect. The dung of cattle, being cohesive,smothers the herbage on which it falls ; but nothing like a per-nicious effect is to be seen around the edges: so far from it, afterthe first rain, the growth is more vigorous; and if the dung beremoved in time, the herbage will be found not to be killed, butblanched by being excluded from the light, and soon recoversits verdure, and becomes more luxuriant than before. Blood andurine, which kill plants, when recently applied, have, in theprocess of their formation, undergone changes, and have gener-ated salts : hence they are injurious to vegetation till these dele-terious compounds are decomposed, which soon happens in thecourse of putrefaction, to which they are prone ; and their inju-rious salts are quickly dissipated in the mass of the dunghill.Nor has it been discovered that the composition of the abovesubstances in the dunghill, is more pernicious than die most in-nocent of its component parts. On the contrary, common ob-servation and experience instruct us, that as soon as putrefactioncommences, the mass is continually yielding a brown extractivematter to the water it contains ; and this liquid is known power-fully to promote vegetatipn, even in the earliest periods of putre-faction, provided it be not too much concentrated, or in too greatquantity.
But it has been said that it is not the juice, but the gasseswhich issue from this composite mass, in the early stages of fer-mentation, by which vegetation is injured. It is admitted thatthese gasses unmixed are indeed hostile to the growth of plantsexposed to their direct effect r but how can that occur to plantsin the field, growing in the open air ? And a mixture of thosegasses, either in the soil or in the air, powerfully promotes vege-tation. Ruckert watered the earth, in which he had plants grow-ing, with water charged with carbonic acid gas, and others withplain water, and found the production of the former to thatof the latter, as 25 to 15. Mr Young made hydrogen gas passthrough the earth in pots in which plants were growing, and