22 REPOKT — 1891. 



not accept the remarkable features brought to light by this photograph 

 as a presumptive indication of a progress of events in cosmical history 

 following broadly upon the lines of Laplace's theory. 



The old view of the original matter of the nebulae, that it consisted of 

 a. ' fiery mist,' 



' a tumultuous cloud 

 Instinct with fire and nitre,' 



fell at once with the rise of the science of thermodynamics. In 1854 

 Helmholtz showed that the supposition of an original fiery condition of 

 the nebulous stuff was unnecessary, since in the mutual gravitation of 

 ■widely separated matter we have a store of potential energy sufficient to 

 generate the high temperature of the sun and stars. We can scarcely go 

 wrong in attributing the light of the nebulce to the conversion of the 

 gravitational energy of shrinkage into molecular motion. 



The idea that the light of comets and of nebulse may be due to a suc- 

 cession of ignited flashes of gas from the encounters of meteoric stones 

 was suo-o-ested by Professor Tait, and was brought to the notice of this 

 Association in 1871 by Sir William Thomson in his Presidential Address. 

 The spectrum of the bright-line nebulje is certainly not such a spec- 

 trum as we should expect from the flashing by collisions of meteorites 

 similar to those which have been analysed in our laboratories. The 

 strongest lines of the substances which in the case of such meteorites 

 would first show themselves, iron, sodium, magnesium, nickel, &c., are 

 not those which distinguish the nebular spectrum. On the contrary, this 

 spectrum is chiefly remarkable for a few brilliant lines, very narrow and 

 defined, upon a background of a faint continuous spectrum, which con- 

 tains numerous bright lines, and probably some lines of absorption. 



The two most conspicuous lines have not been interpreted; for 

 though the second line falls near, it is not coincident with a strong double 

 line of iron. It is hardly necessary to say that though the near position 

 of the brightest line to the bright double line of nitrogen, as seen in a 

 small spectroscope in 1864, naturally suggested at that early time the 

 possibility of the presence of this element in the nebulae, I have been 

 careful to point out, to prevent misapprehension, that in more recent 

 years the nitrogen line and subsequently a lead line have been eruployed 

 by me solely as fiducial points of reference in the spectrum. 



The third line we know to be the second line of the first spectrum of 

 hydrogen. Mr. Keeler has seen the first hydrogen line in the red, and 

 photographs show that this hydrogen spectrum is probably present in its 

 complete form, or nearly so, as we first learnt to know it in the absorp- 

 tion spectrum of the white stars. 



We are not surprised to find associated with it the line D3, near the 

 position of the absent sodium lines, probably due to the atom of some 

 unknown gas, which in the sun can only show itself in the outbursts of 



