Celestial Bodies 145 
dicates that the density of the center must be about that of 
iron, so the probability that the center is composed of iron is 
increased, especially inasmuch as iron is the most cosmically 
abundant heavy metal. Finally, we know that the earth’s 
EARTH. INTERIOR 
temperature rises with depth, from experience with deep 
mines. All of these things together point to the conclusion 
that the center of the earth is composed of molten iron. 
Studies of earthquake waves give further information of 
