188 POTASSIUM, CAUSTIC POTASH, AND CHLORIDE OF POTASSIUM. 



composts, it is not unlikely that a portion of the carbonate of potash may 

 be rendered caustic, and, therefore, be more fit to act upon the vegetable 

 matter in contact with it — by rendering it soluble in water and thus ca- 

 pable of entering into the roots of plants. To this point I shall have 

 occasion to return hereafter. In the mean time, it is proper to remark, 

 that if pearl-ash be mixed, as above prescribed, whh half its weight of 

 quick-lime, and then boiled with less than ten or twelve times its weight 

 of water, a part of the potash only is rendered caustic — the lime being 

 unable to deprive the pearl-ash (carbonate of [wtash) of its carbonic 

 acid, unless it be largely diluted. Hence, in dry composts, or mixtures 

 of this substance with quick-lime, it is unlikely that any large portion of 

 the potash can be at once brought to the caustic state. This fact is 

 really of importance in reference to the theory of the conjoined action of 

 quick-lime and wood or pearl-ash, when mixed together in artificial ma- 

 nures, and applied to the land. 



3°. Potassium. — When dry caustic potash, obtained by evaporating 

 the caustic solution above described, is mixed with powdered charcoal 

 and iron filings, and exposed to an intense heat in an iron retort, it is de- 

 composed, and metallic potassium distils over, and is collected in the 

 form of white shining silvery drops. 



It was one of the most remarkable discoveries of Sir H. Davy, that 

 potash was a compound substance, and consisted of this metal potassium 

 united to oxygen gas. 



Potassium is remarkable for the strong tendency it possesses to unite 

 again with oxygen and re-form potash. When simply exposed to the 

 air, it gradually absorbs oxygen from the atmosphere ; but if it be heat- 

 ed in the air, it takes fire and burns. When the combustion has ceased, 

 a quantity o^ caustic potash remains, the weight of which is nearly one- 

 fifth greater than that of the potassium employed. It even bursts into a 

 flame when thrown upon water, depriving that liquid of its oxygen, and 

 liberating its hydrogen, — and it was justly considered as the most aston- 

 ishing property of this metal, when first discovered, that it took fire 

 v/hen placed upon the coldest ice. [For the composition of water, see 

 Lecture II., p. 36.] When thus burned in contact with water, potash 

 is formed, as before, and is found dissolved in the liquid when the ex- 

 periment is completed. 



4°. Chloride of Potassium. — This is a compound of chlorine with po- 

 tassium, which, in taste, properties, and general appearance, has much 

 resemblance to common salt. It may be formed by dissolving pearl- 

 ash in dilute muriatic acid (spirit of salt) as long as any efTervescence 

 •appears, and afterwards evaporating to dryness. It exists in small 

 ^quantity in sea water, in the ash of most plants, and frequently in the 

 'soil. It is not an article of manufacture, but is occasionally extracted 

 from kelp, and sold to the alum makers. Could it be easily and cheap- 

 ly obtained, there is no doubt that it might be employed with advantage 

 as a manure, and especially in those circumstances in which common 

 salt has been found to promote vegetation. The refuse of the soap-boil- 

 ers, where soap is made from kelp, contains a considerable quantity of 

 this compound. This refuse might be obtained at a cheap rate, and, 

 therefore, might be usefully collected and applied to the land where 

 such works are established. 



