120 ANSWERS TO PRACTICAL QUESTIONS 



24. Why do newly plastered walls remain damp so long? 



The plaster or mortar in drying gives off the water the 

 lime took up in slacking. 



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

 frequent applications ? 



(See Key, page 51, Question 12.) 



26. What causes plaster of Paris to harden again after 

 being moistened ? 



(See Chemistry , page 139.) 



It recombines with the water which was driven off in 

 the process of its manufacture. 



27. What is the difference between sulphate and sulphite 

 of lime'} 



The former is a compound of sulphuric acid ; the lat- 

 ter of sulphurous acid. 



28. What two classes of rays are contained in the mag- 

 nesium light? 



(See Philosophy, page 206.) 



The actinic or chemical, and the colorific or luminous 

 rays. 



29. What rare metals would become useful in the arts, if 

 the process of manufacture were cheapened ? 



Magnesium, aluminum, sodium, etc. 



30. What is the rational formula for calcium carbonate ? 

 Calcium sulphite ? Calcium sulphate ? 



1. CaCO 3 = CaO,CO 2 . 



2. CaSO 3 = CaO,SO 2 . 



3. CaSO 4 = CaO,SO 3 . 



