158 LESSONS IN CHEMISTRY. 



certain vegetable acids are formed, which frequently render 

 the land sour and unfavourable to the growth of useful 

 plants ; much of the beneficial effects of limeing such soils is 

 ascribed to the lime entering into combination with, and 

 neutralizing, these acids. It also promotes the decay of the 

 inert vegetable matters, and thus not only sets free the inor- 

 ganic ingredients which they contain, but disposes then- 

 elements into forms (carbonic acid, &c.) capable of yielding 

 up food to plants. 



246. A compound of sulphur and iron, sulphuret of iron, 

 is found in small quantities in almost all the rocks from which 

 soils are derived ; you must have observed it sparkling in the 

 sunshine, on the face of the clay- slate and basaltic rocks in 

 many parts of the country, like a sprinkhng of brass dust. 

 This compound is insoluble in water, but, by exposure to air 

 and moisture, it unites with oxygen, and is converted into 

 sulphate of iron, "green vitriol" (163), which readily dis- 

 solves, and is found to exercise, when in excess, an injurious 

 effect upon the crops. The addition of carbonate of lime, to 

 soils in which this salt is contained, proves useful by decom- 

 posing it, — gypsum and insoluble peroxide of iron (48) 

 being formed, while the carbonic acid escapes. 



247. Effects of lime upon plants. — The effects of lime in 

 improving the quality of the crops to which it has been 

 applied, have long been remarked ; thus, when applied to old 

 grass-lands, it extirpates coarse and unpalatable plants, and 

 favours the growth of the tender and nutritious white and red 

 clovers. It is said to add to the quantity of gluten pro- 

 duced by the corn-crops, and to increase the weight of the 

 grain; mixed with salt, it gives strength to the straw on 

 mossy lands, where the crops are so frequently lodged. It is 

 also found not merely to improve the quality of almost every 

 crop, rendering the pea more easily boiled and the potato less 

 watery, but it shortens the period of its growth, and hastens 

 the ripening of both grain and roots. 



248. Neither experience nor theory can point out exactly' 

 how much lime should be added to a soil. Professor Johnston 

 states, from the results of his calculations, that in Scotland, at 

 least sufficient should be present to give three per cent to a 

 soil "which contains an ordinary proportion of vegetable 

 matter and the other food of plants," which to one entirely 

 destitute of lime, would, when the soil is twelve inches deep. 



