1865.] Physics. 691 



by the great feebleness which distinguishes the light from the nebulae. 

 A circular portion of the sun's disk subtending 1' would give a light 

 equal to 780 full moons, yet many of the nebulae, though they subtend 

 a much larger angle, are invisible to the naked eye.* Upon the earth, 

 luminous gas emits a light which is very inferior in splendour to 

 incandescent solid matter. 



2. If these enormous masses of gas are luminous throughout, the 

 light from the portions of gas beyond the surface visible to us would 

 be in a great measure extinguished by the absorption of the gas 

 through which it would have to pass. These gaseous nebulae would, 

 therefore, present to us little more than a luminous surface. This 

 consideration may assist in explaining the strange apparent forms of 

 some of the nebulas. 



3. It is probable that two of the constituents of these nebulae are 

 the elements, hydrogen and nitrogen, unless the absence of the other 

 lines of the spectrum of nitrogen indicates a form of matter more 

 elementary than nitrogen. The third gaseous substance is at present 

 unrecognized. 



4. The uniformity and extreme simplicity of the spectra of all 

 these nebulae oppose the opinion that this gaseous matter represents 

 the "nebulous fluid" suggested by Sir William Herschel, out of 

 which stars are elaborated by a process of subsidence and con- 

 densation. In such a primordial fluid all the elements entering into 

 the composition of the stars should be found. If these existed in 

 these nebulae, the spectra of their light would be as crowded with 

 bright lines as the stellar spectra are with dark lines. 



The supposition can scarcely be entertained that the three bright 

 lines indicate a more primary and simple condition of matter ; for 

 then, if the process of elaboration into stars be now taking place, we 

 should expect to find in some of the nebulae, or in some parts of them, 

 a more advanced state towards the formation of the separate elements 

 of which we now know the stars to consist. Such an advance would 

 be indicated by an increased number of bright lines. It is difficult 

 to suppose that the excessively high temperature of the nebulae keeps 

 in check affinities by which, if unrestrained, the formation of the 

 elements would take place ; for in some of the nebulae a nucleus 

 exists, which from its continous spectrum, its greater brightness, and 

 apparent separation from the surrounding gas, we must regard as 

 containing solid or liquid matter. At a temperature at which matter 

 can become liquid or solid (though from peculiar conditions that 

 temperature may be a very exalted one), we cannot suppose the 

 formation of the chemical elements to be restrained by excessive heat. 



5. A progressive formation of some character is suggested by the 

 presence of more condensed portions, and in some nebulae, of a 

 nucleus. Nebulae, which give a continuous spectrum, and yet show 

 but little indication of resolvability, such as the great nebula in 

 Andromeda, are not necessarily clusters of stars. They may be 



* See ' Outlines of Astronomy,' by Sir John F. W. Herschel, p. 616. Seventh 

 edition. 



