What is "rarefies"

Usage examples of "rarefies".

And why should the left cavity of the heart and the great artery be wider and larger than the right cavity and the arterial vein, were it not that the blood of the venous artery, having only been in the lungs after it has passed through the heart, is thinner, and rarefies more readily, and in a higher degree, than the blood which proceeds immediately from the hollow vein?

This atmosphere rarefies rapidly as one ascends the higher peaks, with the result that these are constantly covered with perpetual snow and ice, sending great glaciers down mighty gorges toward the central seas.