224 



plioric acid. Limestones also contain minute quantities of 

 soda probably, in the state of common salt, derived from the 

 salt water of the sea, in which most of our limestones were 

 deposited and perhaps also of potash; but no experiments 

 have yet been made to determine either the proportion of this 

 alkaline matter, or the extent to which that proportion varies in 

 different limestones. 



1. The siliceous ', or earthy matter ', varies from one to forty 

 per cent of the weight of the limestone. When the quantity is 

 very large, the lime, after burning, does not slake or fall to 

 powder on the addition of water. Such limes, after being 

 burned, are usually crushed by machinery, and are then used 

 as cements. They set or harden almost immediately when 

 mixed with water, but- are never employed for agricultural 

 purposes. 



When a limestone containing silica is burned in a kiln, the 

 silica combines with a portion of the caustic lime, and forms 

 silicate of lime. The presence of this silicate must modify the 

 action of the lime upon the land to which it is applied ; and the 

 extent of the modification must be commensurate with the pro- 

 portion of siliceous matter which the limestone originally con- 

 tained. In what way, or to what extent, the silicate of lime 

 modifies the special action of the caustic lime, has not as yet 

 been made the subject of experiment. 



2. Magnesia exists in nearly all limestones in greater or less 

 proportion. Good agricultural limes rarely contain more than 

 five per cent ; but in what are called magnesian limes, or dolo- 

 mites, the magnesia sometimes forms as much as one-half of the 

 whole weight. Such limes are usually considered to be inju- 

 rious to the land. They are said to be too caustic, to burn it 

 up, and to destroy the crops. Precise experiments, made upon 

 different crops, are much wanted to bring out clearly the special 

 action of the magnesian ingredient of these limes, and to what 

 extent it may be safely applied to given crops and soils. 



3. Sulphur exists in all our limes in minute quantity in the 

 state of sulphuric acid, forming with the lime sulphate of lime 

 or gypsum. The quantity, so far as analyses have yet been 

 made, varies from one-third of a per cent to one per cent of 



