CALCIUM AND ITS COMPOUNDS 385 



high temperature causes the impurities in the limestone (the clay, 

 etc.) to interact with the quicklime and form fusible silicates, 

 which fill the pores and retard the subsequent action of water in 

 slaking the lime. Calcium carbonate gives a pressure of only 

 25 mm. of carbon dioxide at 700 and one atmosphere at about 

 900. The action is reversible (see equation), and if the gas ac- 

 cumulates in the kiln, the carbon dioxide recombines with the 

 quicklime as fast as it is liberated unless a temperature above 

 900 is used. When, however, the gas is continually removed, 

 the backward action is prevented, and a lower temperature suf- 

 fices to complete the dissociation of the compound. Hence a 

 low temperature and a good draft of air through the kiln are 

 essential. 



Properties and Uses of Quicklime. Calcium oxide is a 

 white, amorphous material. It melts only in the electric arc. 

 When heated strongly, it glows with an unusually brilliant and 

 white light. The Drummond, or oxy-hydrogen light, more com- 

 monly called the lime light or calcium light, is produced by al- 

 lowing a flame of burning oxygen and hydrogen or illuminating 

 gas to play upon a cylinder of calcium 

 oxide (Fig. 96). The gases are contained 

 in iron cylinders, under pressure, and so 

 the apparatus can be used for illumination 

 or projection where neither electricity nor FlG 



a local supply of gas is available. 



When water is poured upon quicklime, it is at first absorbed, 

 and then enters into combination to form calcium hydroxide 

 (slaked lime) : 



CaO + H 2 - Ca(OH) 2 . 



So much heat is given out that the excess of water is converted 

 into steam. The quicklime swells and falls to powder. 



Quicklime is used most largely in making slaked lime for mor- 



