[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:258|3|0]]area it is difficult to see how the separate particles could all act in unison, as they appear to do.

Allusion should be made to the theory recently put forward both by Prof. Newall and Mr G. Burns (possibly by others also) that the tail of a comet consists of matter already in situ, but in some way excited to luminosity by the passage of the comet. They point out that this would explain the similarity of spectrum shown by most comets’ tails, and Prof. Newall has found evidence of the presence of cyanogen in the interplanetary spaces. The idea of scattered gases is supported by the occasional shattering of comets’ tails; instances are Brooks’ comet photographed by Barnard in 1893, and Morehouse’s comet on Oct. 15, 1908; there can scarcely be any doubt that the tail matter met some obstruction in space, which suddenly checked its onward movement.

But I find it difficult to believe that, at least, the beginning of the tail is not an emission from the head, since spectrograms with an objective prism frequently show head and tail absolutely continuous, so that it is not easy to say where the one passes into the other.

I pass now to the idea of the tail being due to the ejection of material particles from the sun. I think that Prof. Schaeberle was the first to put forward this idea in 1893. He thought that both the corona and comets’ tails are formed in this manner, his words in «Astronomical Journal», 306, being «The tail of a comet is produced by the visible particles of matter originally forming the comet’s atmosphere, and by the previously invisible particles of a coronal stream which moving with great velocity, finally produce by repeated impact of the successive particles almost the same motions in the visible atmosphere of the comet as would be communicated by a continuously accelerating force directed away from the sun.... The coronal matter, owing to its retardation, grows so dense that it also becomes visible, and with the comet’s atmosphere is finally driven into the tail by the repeated bombardment of unretarded following portions of the stream». Since this was written, the radio-active elements have been discovered, and the theory of the dissociation of the atom into electrons has been adopted. Hence the particles suggested by Prof. Newall and Mr G. Burns are