Horticultural Chemistry. 79 



use. Yet I do not think such a plan would answer in a 

 country like England, where the pi'inciple of liberty is the 

 foundation of all your institutions ; but I know every thing 

 connected with horticulture is interesting to you. In 1823, 1 

 was examined, and gained the best character. 

 I remain, dear Sir, &c. 



Jens Peter Petersen. 

 Copenhagen, Dec. 29. 1827. 



Art. III. Outlines of Horticultural Chemistry: — Manures. 

 By G. W. Johnston, Esq., of Great Totham, Essex. 



(^Continued from Vol. III. p. 404.) 



Some manures ameliorate a soil by absorbing moisture 

 from the atmosphere. This property is at least as beneficial 

 to ground that is aluminous as to that which is siliceous ; for 

 it is equally useless to either, during such periods of the 

 year as are characterised by a plentiful deposition of rain, 

 but in the droughts of summer, when moisture is much 

 wanting to plants, it is beneficial to both : in very dry sea- 

 sons it is even of greater importance to clayey soils than to 

 light ones ; for vegetation on the former suffers more from long- 

 continued drought than on the latter, inasmuch as that mois- 

 ture being equally exhaled from each, the surface of the 

 clayey soil becomes caked and impervious to the air, which is 

 the only grand source of compensatory moisture that is avail- 

 able to the languishing plants, and which is more open to 

 those which grow on light, and consequently more pervious 

 soils. 



The following table of the comparative absorbent powers 

 of many manures, &c., is chiefly extracted from An Essay on 

 the Uses of Salt in Agriculture, Sfc, by my brother, Mr. C. 

 Johnston : — 



Parts 

 'Horse dung, evaoorated previously to dryness, at a temperature 

 of 100°, absorbed during an exposure of three hours to air 

 saturated with moisture at 62° - - - 145 



Putrefied tanner's bark, under similar circumstances, {66^) - 145 

 Unputrefied ditto - - - - -115 



Cow dung - - - - - 130 



Pig ditto ^ - - - - - 120 



Sheep ditto - - - - - 8 1 



o-^ Pigeon ditto - _ - - - 50 



§ I Refuse marine salt (60°) - - - - 49^ 



2 I Soot (68°) - ... - 36 



