180 



SIR H. DAVY S AGRICULTURAL CHE:»IISTRY. 



such results as are useful to the agricul- 

 turist may be easily obtained. 



The apparatus necessary, is a green 

 glass retort, attached by cement to a re- 

 ceiver, connected with a tube passing un- 

 der an inverted jar of known capacity, 

 filled with water. A given weight of 

 the substance is to be heated to redness 

 in the retort over a charcoal fire; the re- 

 ceiver is to be kept cool, and the process 

 continued as long as any elastic matter is 

 generated. The condcnsible fluids will 



be formed of the composition of the sub- 

 stance. The proportions of the elements 

 in the greater number of the vegetable 

 substances which can be used as food, 

 have been already ascertained by philo- 

 sophical chemists, and have been stated 

 in the preceding pages; the analysis by 

 distillation may, however, in some cases, 

 be useful in estimating the powers of 

 manures in a manner that will be ex- 

 plained in a future lecture. 



The statements of the composition of 



collect in the receiver, and the fixed resi- 1 vegetable substances, quoted from MM. 



duum will be found in the retort. The 

 fluid products of the distillation of vege- 

 table substances are principally water, 

 with some acetous and mucous acids and 

 empyreumatic oil, or tar, and in some 

 cases ammonia. The gassesare carbonic 

 acid gas, carbonic oxide, and carburetted 

 hydrogen; sometimes with olefiant gas, 

 and hydrogen; and sometimes, but more 

 rarely, with azote. Carbonic acid is the 

 only one of those gasses rapidly absorb- 

 ed by water; the rest are inflammable; 

 olefiant gas burns with a bright white 

 light; carburetted hydrogen with a light 

 like wax; carbonic oxide with a feeble, 

 blue flame. The properties of hydrogen 

 and azote have been described in the last 

 lecture. The specific gravity of carbonic 

 acid gas, is to that of air as 20.7 to 13.7, 

 and it consists of one proportion of c?r- 

 bon 11.4, and two of oxygen 30. The 

 specific gravity of gaseous oxide of car- 

 bon, is, taking the same standard, 13.2, 

 and it consists of one proportion of car- 

 bon, and one of oxygen. The specific 

 gravities of carburetted hydrogen and 

 olefiant gas are respectively S and 13; 

 both contain four proportions of hydro- 

 gen; the first contains one proportion, 

 the second two proportions of carbon. 

 If the weight of the carbonaceous resi- 

 duum be added to the weight of the fluids 

 condensed in the receiver, and they be^ 

 subtracted from the whole weight of the 

 substance, the remainder will be the 

 weight of the gasseous matter. 



The acetous and nmcous acids, and the 

 ammonia formed, are usually in very 

 small quantities; and by comparing the 

 proportions of water and charcoal with 

 the quantity of the gasses, taking into ac- 

 count their qualities, a general idea may 



Gay Lussac and Thenard were obtained 

 by these philosophers by exposing the 

 substances to the action of heated hyper- 

 oxymuriate of potassa; a body that con- 

 sists of potassium, chlorine, and oxygen, 

 and which aff'orded oxygen to the carbon 

 and the hydrogen. Their experiments 

 were made in a peculiar apparatus, and 

 required great caution, and were of a very 

 delicate nature. It will not therefore be 

 necessary to enter upon any details of 

 them. 



It is evident from the whole tenor of 

 the statements which have been made, 

 that the most essential vegetable sub- 

 stances consist of hydrogen, carbon, and 

 oxygen indifferent proportions, generally 

 alone, but in some few cases combined with 

 azotes. The acids, alkalies, earths, metallic 

 oxides, and saline compounds, though 

 necessary in the vegetable economy, 

 must be considered as of less importance, 

 particularly in their relation to agricul- 

 ture, than the other principles: and as it 

 appears from M. de Saussure's table, and 

 from other experiments, they differ in the 

 same species of vegetable when it is rais- 

 ed on different soils. 



MM. Gay Lussac and Thenard have 

 deduced three propositions, which they 

 have called laivs, from. their experiments 

 on vegetable substances. The first is, 

 " that a vegetable substance is always 

 acid, whenever the oxygen it contains is 

 to the hydrogen in a greater proportion 

 than in water." 



The second, "that a vegetable sub- 

 stance is always resinous or oily or 

 spirituous, whenever it contains oxygen 

 in a smaller proportion to the hydrogen 

 than exists in the water." 



The third, "that a vegetable sub- 



