Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Humphry Davy
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

    255/318 (page 239)
    of dung which have been mentioned, there seems no reason why it should be made to ferment except in the soil; or if suffered to ferment, it should be only in a very slight degree. The grass in the neighbourhood of re- cently voided dung, is always coarse and dark green ; some persons have attributed this to a noxious quality in unfermented dung ; but it seems to be rather the result of an excess of food furnished to the plants. The question of the proper mode of the application of the dung of horses and cattle, however, properly belongs to the subject of composite manures, for it is usually mixed in the farm-yard with straw, offal, chaff, and various kinds of litter; and itself contains a large proportion of fibrous vegetable matter. A slight incipient fermentation is undoubtedly of use in the dunghill; for by means of it a disposition is brought on in the woody fibre to decay and dissolve, when it is carried to the land, or ploughed into the soil; and woody fibre is always in great excess in the refuse of the farm. Too great a degree of fermentation is, however, very prejudicial to the composite manure in the dunghill; it is better that there should be no fermentation at all before the manure is used, than that it should be car- ried too far. This must be obvious from what has been already stated in this Lecture. The excess of fermentation tends to the destruction and dissi- pation of the most useful part of the manure; and the ultimate results of this process are like those of combustion. It is a common practice amongst farmers, to suffer the farm-yard dung to ferment till the fibrous texture of the vegetable matter is entirely broken down; and till the manure becomes perfectly cold, and so soft as to be easily cut by the spade. Independent of the general theoretical views unfavourable to this prac- tice founded upon the nature and composition of vegetable substances, there are many arguments and facts which shew that it is prejudicial to the interests of the farmer. During the violent fermentation which is necessary for reducing farm- yard manure to the state in which it is called short muck, not only a large quantity of fluid, but likewise of gaseous matter is lost; so much so, that the dung is reduced one-half, or two-thirds in weight; and the principal elastic matter disengaged, is carbonic acid with some ammonia; and both these, if retained by the moisture in the soil, as has been stated before, are capable of becoming an useful nourishment of plants. In October, 1808, I filled a large retort capable of containing three pints of water, with some hot fermenting manure, consisting principally of the litter and dung of cattle; I adapted a small receiver to the retort, and connected the whole with a mercurial pneumatic apparatus, so as to collect the condensible and elastic fluids which might rise from the dung. The receiver soon became lined with dew, and drops began in a few hours to trickle down the sides of it. Elastic fluid likewise was generated; in three days 35 cubical inches had been formed, which, when analysed, were found to contain 21 cubical inches of carbonic acid, the remainder was hydrocarbonate mixed with some azote, probably no more than existed in the common air in the receiver. The fluid matter collected in the receiver at the same time amounted to nearly half an ounce. It had a saline taste, and a disagreeable smell, and contained some acetate and carbonate of ammonia. Finding such products given off from fermenting litter, I introduced the beak of another retort filled with similar dung very hot at the time, into the soil amongst the roots of some grass in the border of a garden