Here, then, is the means of determining whether the light emitted by a given nebula comes from an incandescent gas, or from a congeries of ignited solids, stars, or suns.

If its spectrum be discontinuous, it is a true nebula or gas; if continuous, a congeries of stars.
In 1864, Mr.Huggins made this examination in the case of a nebula in the constellation Draco.

It proved to be gaseous.
Subsequent observations have shown that, of sixty nebulae examined, nineteen give discontinuous or gaseous spectra--the remainder continuous ones.
It may, therefore, be admitted that physical evidence has at length been obtained, demonstrating the existence of vast masses of matter in a gaseous condition, and at a temperature of incandescence.

The hypothesis of Laplace has thus a firm basis.

In such a nebular mass, cooling by radiation is a necessary incident, and condensation and rotation the inevitable results.