the ordinary gases have assumed extreme tenuity, and the lighter gas becomes of more relative importance, and gives a character to the spectrum.
As it is instructive to learn, as far as may be, the boundary between the known and the unknown, it is interesting to read what Sir W. Huggins has to tell us about the solar corona. The nature of this marvellous appendage to the sun is still a matter of uncertainty. There can, however, be no doubt that the corona consists of highly attenuated matter driven outwards from the sun by some repulsive force, and it is also clear that if this force be not electric it must at least be something of a very kindred character. Dr. Schuster suggests that there may be an electric connection between the sun and the planets. In fact, with some limitations we might even assert there must be such a connection. It is well known that great outbreaks on the sun have been immediately followed, I might almost say accompanied, by remarkable magnetic disturbances on the earth. The instances that are recorded of this connection are altogether too remarkable to be set aside as coincidences. Sir William Huggins has not referred in this connection to Hertz's astonishing discoveries; but it seems quite possible that research along this line may throw much light on the subject, at present so obscure, of the electric relation between the sun and the earth. So far as the spectrum of the corona is concerned we may summarise what is known in the words of Huggins: "The green coronal line has no known representative in terrestrial substances, nor has Schuster been able to recognise any of our elements in the other lines of the corona."
Sir William Huggins regarded it as surprising that our
