232 Precipitation of one Earth by Another 



bonic acid ; does it not equally account for the absence of 

 this substance in the waters of the sea, and in all waters ex- 

 posed to the au', running over or resting on calcareous, mag- 

 nesian, or aluminous rocks \ Owing to the readiness with 

 which it is thrown down from its saline solutions, it is easy 

 to comprehend its being collected in nodules and layers, and 

 abundantly associated, as it so often is, with earths fitted, 

 when it is taken to be applied to the use of man, to serve as 

 a flux in the fusion and reduction of its ores. Owing to the 

 same circumstance, viz., the facility with which it is precipi- 

 tated ; whether by the action of lime, or magnesia, or alu- 

 mine, separating it from an acid, or simply by the absorption 

 of oxygen, and the conversion of the protoxide into the per- 

 oxide, and the disengagement of carbonic acid ; it is not diffi- 

 cult for us to understand how this oxide, the peroxide, is the 

 colouring matter almost universally of the earth's surface, 

 imparting various tints of brown and yellow, admirably har- 

 monizing with the colouring derived from plants, whether in 

 its greatest purity, as on the surface of limestone rock, itself 

 Avhite, giving tints rivalling almost the hue and richness of 

 gold, such as we see in the sea-cliifs and mountain precipices 

 of calcareous countries, especially of Greece ; or, in less 

 purity mixed with diiFerent proportions of vegetable matter 

 in a state of decay, giving to the soil a soberer and more suit- 

 able hue for an expanded surface in almost endless variety 

 of shades of red and brown. 



What has been stated of phosphate of lime seems very de- 

 serving of consideration, in connection with vegetable an^ 

 perhaps animal physiology. Does not the precipitation of 

 this phosphate, in the manner mentioned, from acids, tend to 

 preserve a store of it on our lands .? Were it otherwise, would 

 not the greater part of it soon be washed into the ocean, and 

 be lost to man^ And then its solubility in water impreg- 

 nated with a gaseous acid — ^the carbonic — always tending to 

 pass off into the atmosphere, affords great facility to its being 

 conveyed from the soil, and deposited where required in the 

 structure of plants during their process of growth. 



I have adverted to the solubility of phosphate of lime by 

 the acetic acid, and even when this acid is much diluted, as 



