BOCK WEATHERING 



83 



locality, where the weather conditions are different, will 

 crumble rapidly. A striking example of this is found in 

 the great stone obelisk, called Cleopatra's Needle, which 

 was brought from Egypt to Central Park, New York, 

 some time ago. Although it had stood for 3000 years in 



Egypt without losing 



the distinctness of the 

 carving upon it, yet in 

 the moist and change- 

 able climate of New 

 York it was found nec- 

 essary within a year to 

 cover its surface with a 

 preservative substance. 

 Not only do different 

 climates affect differ- 

 ently the wearing away 

 of rocks, but different 

 kinds of rocks them- 

 selves vary much in 

 the rate at which they 

 crumble. It has been 

 found that while mar- 

 ble inscriptions, in a 

 large town where there 



is much COal smoke and CLEOPATRA'S NEEDLE, CENTRAL PARK, 

 . , , , . . NEW YORK. 



considerable ram, will 



become illegible in fifty years, that after a hundred years 



inscriptions cut in slate are sharp and distinct. 



Experiment 45. Allow a test tube filled with water and tightly 

 corked to freeze. What happens ? If the temperature of the air is 

 not cold enough, place the test tube in a mixture of chopped ice and 

 salt, or better, chopped ice and ammonium chloride (sal ammoniac), 

 and allow it to remain for some time. 



