The spectroscope shows that these materials-hydrogen, hydrocarbons, and iron-are present in comets, and leaves little room for doubt of the essential soundness of Bredichin's theory.
"A Text-Book of Astronomy"
George C. Comstock
Bredichin concludes that the straightest of the three tails, marked as Type I., is most probably due to the element hydrogen; the tails of the second form are due to the presence of some of the hydrocarbons in the body of the comet; while the small tails of the third type may be due to iron or to some other element with a high atomic weight.
"The Story of the Heavens"
Robert Stawell Ball
Is it the C which burns, or the hydrocarbons?
"An Introduction to Chemical Science"
R.P. Williams