108 THE CULTURE OF FARM CROPS. 



a combination of potash with carbonic acid. This is the 

 form in which potash exists in wood ashes ; and in the pot- 

 ash and pearl ash of commerce. It has a most important 

 influence upon the growth of plants, as may be seen by 

 reference to the tables in the previous chapter. Its use 

 for this purpose as a fertilizer dates back to the time of the 

 ancient Hebrews, Egyptians, and Romans, and the value 

 of wood ashes as a fertilizer has been mentioned by several 

 of the ancient writers. Moreover it is well known that 

 wood ashes are more favorable to some plants than to oth- 

 ers, "bringing in," as it is termed, plants like the clovers 

 which are rich in potash, and so crowding out useless weeds, 

 and improving the land at the same time. 



POTASH is extremely caustic, destroying all vegetable and 

 animal matter very rapidly. It is easily produced as fol- 

 lows. 12 parts by weight of carbonate of potash are dissolv- 

 ed in water and boiled with half the weight of newly 

 burned (or quick or caustic) lime slaked in water, the lime 

 takes the carbonic acid from the potash and settles to the 

 bottom, leaving the potash in solution in a caustic state. 

 Caustic potash so readily absorbs water, from the atmos- 

 phere, that it can only be kept dry with difficulty. It is 

 not known that potash in this form is of any service in the 

 growth of plants, but it is thought possible, because of the 

 action of lime upon the carbonate; and when lime is ap- 

 plied to the soil, as it frequently is, it is quite possible that 

 it may exert this effect upon the soluble carbonate of pot- 

 ash with which it comes in contact. 



POTASSIUM, may be obtained by mixing the dry caustic 

 potash, procured by evaporating the solution above de- 

 scribed to dryness, with powdered charcoal and iron filings, 

 and submitting the mass to intense heat in a closed retort. 

 The potash is decomposed; its oxygen combines with the 

 iron, and the metal potassium is left pure in the form of a 

 vapor which is distilled over and appears, on cooling, in 

 the form of white silvery drops. This process was one of 

 the remarkable discoveries of Sir Humphrey Davy to whom 

 we are indebted for much that is known of agricultural 



