DR. DONALD H. MENZEL has an article entitled “What is the Solar Corona ?”in the Telescope of May–June, in which he refers specially to the researches of Bengt Edlén, of Uppsala, who has shown that coronium is chiefly iron. Nickel and calcium have also baen identified, the outer electrons in all these elements being torn away. Difficulties arise in explaining how the comparatively low temperature of the sun—about 6,000° C.—can be responsible for tearing away the outer electrons, the removal of which requires a temperature of at least 100,000° C. Then again, Edlén has pointed out that the great breadth of the coronal lines suggests a very rapid movement of the atoms, and a temperature of 2,000,000° C. would be required to explain this. Conclusions of a similar nature have been reached by independent lines of investigation and various explanations have been suggested to account for the source of this high temperature. The most acceptable hypothesis is that the highly heated coronal matter is issuing in great jets from holes and cracks in the solar surface. These crevices, which are probably associated with sunspots, run far down into the hot interior, where the temperature is several million degrees.