NATURE AND ACTION OF MANURES. 49 



giving out an aqueous smoke, and leaving a dry white 

 residuum. But when the salts are insoluble, as phos- 

 phate of lime ; or soluble with difficulty, as sulphate of 

 lime, water will not act upon them, and they remain 

 mixed with the earth without their existence being sus- 

 pected, as long as an analysis is confined to the limits I 

 have laid down. However, these substances, especially 

 the sulphate of lime, influence so much the quality of 

 soils, that it is necessary to furnish the means for ascer- 

 taining their existence, I will however observe, that 

 these salts are contained in the earths in so small a 

 quantity, as not to influence sensibly the results of the 

 analysis I have directed, for ascertaining the natures and 

 proportions of the other principles which essentially com- 

 pose them. 



To ascertain if a soil contains sulphate of lime, (gyp- 

 sum, plaster of Paris,) take an exact quantity, four hun- 

 dred grains for example, mixed with one third the quanr 

 tity of powdered charcoal, expose it in a crucible during 

 half an hour to a red heat ; afterwards boil it for a quar- 

 ter of an hour in half a pint of water, filter the liquor, 

 and expose it for some days in an open vessel ; if it form 

 a white precipitate, the soil contains sulphate of lime, 

 and the weight of the deposit will make known nearly 

 the proportion. (Davy.) 



To judge of the existence of phosphate of lime, digest 

 the earth in an excess of muriatic acid, evaporate the 

 solution to dryness, wash the residuum in a large quan- 

 tity of water, and the insoluble phosphate will remain 

 alone. 



CHAPTER HI 



OF THE NATURE AND ACTION OF MANURES. 



Under the general head of manures are comprehended 

 all those substances which, existing in the atmosphere or 

 combining with the soil, can be drawn in by the organs 

 of plants, and contribute to the progress of vegetation. 



Manures are furnished by various bodies belonging to 

 the three kingdoms of nature. Those most common!/ 

 5 



