AY POPULAR CHEMISTRY. 91 



25. Will lime lose its beneficial effect upon a soil after 

 frequent applications ? 



Lime acts in various ways to improve the fertility of a soil. 

 It corrects its acidity, aids in the decomposition of the rocky 

 constituents, hastens the decay of the humus, and also makes 

 the soil more -porous. It does not, however, benefit the grow- 

 ing plant directly, but works up other materials in the soil. It 

 therefore loses its effect after a time. The Belgian farmers 

 have a proverb : 



" Much, lime and no manure, 

 Make farm and farmer poorer." 



26. What causes plaster of Paris to harden again after 

 being moistened ? 



(Seo Chemistry, p. 141.) 



It recombines with water, which was driven off in the 

 process of its manufacture. 



27 What is the difference between sulphate and sul- 

 phite of lime? 



The former is a compound of sulphuric acid ; the latter of 

 sulphurous acid. 



28. What two classes of rays are especially abundant 

 in the magnesium light ? 



(See Popular Physics, p. 243.) 



The actinic and the luminous rays. The former are less than 

 ^u^Tnr of an mcn m length, and produce chemical change. 



29. What rare metals would become useful in the arts 

 if the process of manufacture were cheapened ? 



Magnesium, aluminum, sodium, etc. 



30. Why is lime placed in the bottom of a leach-tub ? 



The potash of the ashes is generally in the form of a car- 

 bonate, the acid neutralizing in part the strength of the alkali. 

 The lime combines with the CO 2 . 



31. Is saleratus a salt of IL or of Na? 



It should be a carbonate of K, but, on account of its 

 cheapness, the corresponding salt of Na is often sold instead. 



