The more cautious astronomers believe that chemistry is not sufficiently advanced to justify all of these deductions; that, until chemists have settled the lately raised question of the transmutation of elements, no theory can be sure. It is also held that until they have explained, without room for doubt, the reasons for the presence of some lines, and the absence of others, of any element in a stellar spectrum; why the arc-spectrum of each element differs from its spark spectrum; what are all the various changes produced in the spectrum of a gas by all possible concomitant variations of pressure and temperature; also the meanings of all the flutings in the spectra of metalloids and compounds; and other equally pertinent matters—until that time arrives the part to be played by the astronomer is one of observation. By all means, they say, make use of “working hypotheses” to add an interest to years of laborious research, and to serve as a guide to the direction of further labours; but be sure not to fall into the error of calling any mere hypothesis a theory.
Nebular Hypothesis.—The Nebular Hypothesis, which was first, as it were, tentatively put forward by Laplace as a note in his Système du Monde, supposes the solar system to have been a flat, disk-shaped nebula at a high temperature in rapid rotation. In cooling it condensed, leaving revolving rings at different distances from the centre. These themselves were supposed to condense into the nucleus for a rotating planet, which might, in contracting, again throw off rings to form satellites. The speculation can be put in a really attractive form, but is in direct opposition to many of the actual facts; and so long as it is not favoured by those who wish to maintain the position of astronomy as the most exact of the sciences—exact in its facts, exact in its logic—this speculation must be recorded by the historian, only as he records the guesses of the ancient Greeks--as an interesting phase in the history of human thought.
Other hypotheses, having the same end in view, are the meteoritic hypothesis of Lockyer and the planetesimal hypothesis that has been largely developed in the United States. These can best be read in the original papers to various journals, references to which may be found in the footnotes of Miss Clerke’s History of Astronomy during the Nineteenth Century. The same can be said of Bredichin’s hypothesis of comets’ tails, Arrhenius’s book on the applications of the theory of light repulsion, the speculations on radium, the origin of the sun’s heat and the age of the earth, the electron hypothesis of terrestrial magnetism, and a host of similar speculations, all combining to throw an interesting light on the evolution of a modern train of thought that seems to delight in conjecture, while rebelling against that strict mathematical logic which has crowned astronomy as the queen of the sciences.
FOOTNOTES:
[1] R. S. Phil Trans., 1810 and 1817-24.
[2] One of the most valuable contributions to our knowledge of stellar parallaxes is the result of Gill’s work (Cape Results, vol. iii., part ii., 1900).
[3] Taking the velocity of light at 186,000 miles a second, and the earth’s mean distance at 93,000,000 miles, 1 light-year=5,865,696,000,000 miles or 63,072 astronomical units; 1 astronomical unit a year=2.94 miles a second; and the earth’s orbital velocity=18.5 miles a second.
[4] Ast. Nacht., 1889.
[5] R. S. Phil. Trans., 1718.