66 A YEAK IN SCIENCE 



mined by the clearness of its crystals which refract 

 the light in passing through it into brilliant colors. 

 The refractive power of the diamond is increased by 

 cutting its surface into numerous facets. Being the 

 hardest substance known, it takes diamond to cut dia- 

 mond, so this process is accomplished by grinding tho 

 stone to be cut with black and imperfect pieces of 

 diamond which are valueless as gems. 



Diamonds which are valueless as jewels are of much 

 value in pointing glass cutters and in making drills 

 to drill rocks. 



The most valuable diamond mines are the Kimberly 

 mines of South Africa, though the stone is found in 

 smaller deposits in South America and Australia. 



Carbon dioxide = C0 2 . Burning is a phenomenon 

 familiar to all and yet in the process many chemical 

 changes may be taking place. The visible results are 

 that the object burned is separated into smoke, flame, 

 and ash, together with the heat given off. Another 

 product of most burning is an invisible gas called 

 carbon dioxide. This gas is formed by the union of 

 the oxygen of the air with the carbon in the substance. 

 It may be expressed as C + 2 X = C0 2 in which two 

 parts of oxygen combine with one part of carbon to 

 form carbon dioxide. It is constantly emitted wherever 

 there are living organisms, for all living forms "burn" 

 carbon in the body, and the carbon dioxide is given off 

 in breathing. Even in decay, plant and animal forms 

 give off this gas. 



