MINERALOGY. 303 



the level of the sea does not regulate its temperature 

 as it does that of the surface. Water, in the Kil- 

 Hngsworth Colliery, at the depth of 1200 feet under 

 the level of the sea, was 74 Fahrenheit, while the 

 air at 436 deep, in the mine of Villapenda, in Mexico, 

 which is more than 3000 feet above the level of the 

 sea, was 85. In the Mina Purgatoria, the height 

 of which, above the level of the sea, is equal to the 

 Peak of Teneriffe. the air of the mine was 67. 



Diamonds. 



The weight of diamonds is estimated by carats, 150 

 of which are equal to an ounce troy, or 480 grains. 



The difference of value between one diamond and 

 another is, generally speaking, as the squares of 

 their respective weights. Thus, the value of three 

 diamonds, of 1 , 2, or 3 carats weight respectively, is 

 as 1,4, and 9. 



The average price of rough diamonds is estimated 

 by Jeffries at 2 per carat, and consequently, when 

 wrought, the cost of the first carat, exclusive of 

 workmanship, will be ,8, which is the value of a 

 rough diamond of two carats. A wrought diamond 

 of three carats is worth .72 ; of four carats, 128 ; 

 of forty carats, 12,000 ; and of one hundred carats, 

 80,000. .,>; 



The Empress Catherine's diamond weighed 195 

 carats, and it was purchased by her for 90,000 

 cash, and an annuity of 4000. The Pitt diamond 

 is the most perfect and beautiful yet known. It 

 weighs 136 carats, and was purchased by the Regent 

 Duke of Orleans for 100,000. 



Alumina. 



The earth, alumina, (clay,) constitutes some of the 

 hardest gems, such as the sapphire and ruby ; but it 

 rarely occurs quite pure. Its presence is known by 



