STATES IN WHICH LIME IS APPLIED. 3b9 



forms a hydrate only, without absorbing any sensible quantity of car- 

 bonic acid. The hydrate thus produced is met with in the form of 

 mineral deposits on various parts of the earth's surface, and this mineral 

 is not known to undergo any change or to absorb carbonic acid though 

 exposed for a great length of time to the air. When magnesian limes 

 are slaked by water, therefore, the magnesia they contain may remain 

 in whole or ui part in the caustic state (of hydrate), which will change 

 but slowly even when exposed to the air. When it is left to sponta- 

 neous slaking, one-fourlh of it at least will always remain in the caustic 

 state, however long it may be exposed to the air. 



Should a lime be naturally of such a kind, or be so mixed with the 

 ingredients of the soil as to form a hydraulic cement or an ordinary 

 mortar, which will solidify when rains come upon it, or when the natu- 

 ral moisture of the soil reaches it — the absorption of carbonic acid will 

 in a great measure cease as it becomes solid, and a large proportion of 

 the lime will remain caustic for an indefinite period. 



§ 4. Slates of chemical combination in which lime may be applied to 

 the land. 



There are, therefore, four distinct slates of chemical combination, in 

 which pure lime may be artificially applied to the land. 



1^. Quick-lime or lime-shells, in which the lime as it comes from the 

 kiln is uncombined either with water or with carbonic acid. 



2°. SlaJced lime or hydrate of lime, in which by the direct application 

 of water it has been made to combine with about one-fourth of its 

 weight of water. 



In both these states the lime is caustic, and may be properly spoken 

 of as caustic lime. 



3°. Spontaneously slaked liine,m which one-half of the lime is com- 

 bined with water and the other half with carbonic acid. In this state 

 it is only half caustic. 



4°. Carbonate of lime — the state in which it occurs in nature, and to 

 which burned lime, after long exposure to the air, more or less perfectly 

 arrives. In this stale lime possesses no caustic or alkaline (p. 48, § 5< 

 note) properties, but is properly called mild lime. 



5°. l^i-carbonale of lime may be adverted to as a fifth state of com- 

 bination, in which, as I have previously explained to you (pp. 45-6, 

 § 1), nature usually applies lime to the land. In this state it is combined 

 with a double proportion of carbonic acid, and is to a certain extent 

 readily soluble in water. Hence, springs are often impregnated with 

 it, and the waters that gush from fissures in the lime-stone rocks spread 

 it through the soil in their neighbourhood, and sweeten the land. 



1 shall hereafter speak of these several states under the names of 

 quick-Wme, hydrate of lime, spontaneously slaked lime, carbonate of 

 lime, and Bi-carbonate of lime. By adhering to these strictly correct 

 names, we shall avoid some of that confusion into which those who 

 have hitherto treated of the use of lime as a manure have unavoidably- 

 fallen. The term mild, you will understand, applies only to that which 

 is entirely in the state of carbonate. 



Magnesia, in the magnesian limes, may in like manner be either in 

 the state o{ calcined magnesia, o^ hydrate of magnesia^ oi spontaneously 



