of some of the Nebula. 531 



Such gaseous masses would be doubtless, from many causes, 

 unequally dense in different portions ; and if matter condensed 

 into the liquid or solid state were also present, it would, from its 

 superior splendour, be visible as a bright point or points within 

 the disk of the nebula. These suggestions are in close accord- 

 ance with the observations of Lord Rosse. 



Another consideration which opposes the notion that these 

 nebulae are clusters of stars is found in the extreme simplicity of 

 constitution which the three bright lines suggest, whether or 

 not we regard these lines as indicating the presence of nitrogen, 

 hydrogen, and a substance unknown. 



It is perhaps of importance to state that, except nitrogen, no 

 one of thirty of the chemical elements the spectra of which I have 

 measured has a strong line very near the bright line of the ne- 

 bulae. If, however, this line were due to nitrogen, we ought to 

 see other lines as well ; for there are specially two strong double 

 lines in the spectrum of nitrogen, one at least of which, if they 

 existed in the light of the nebulae, would be easily visible*. In 

 my experiments on the spectrum of nitrogen, I found that the 

 character of the brightest of the lines of nitrogen, that with 

 which the line in the nebulae coincides, differs from that of the 

 two double lines next in brilliancy. This line is more nebulous 

 at the edges, even when the slit is narrow and the other lines are 

 thin and sharp. The same phenomenon was observed with some 

 of the other elements f. We do not yet know the origin of this 

 difference of character observable among lines of the same ele- 

 ment. May it not indicate a physical difference in the atoms, 



nebulae have been obtained in terms of the light of a sperm candle burning 

 at the rate of 158 grains per hour. 



The light of nebula 4628, 1 H. IV. = T ^ part of that of candle. 

 „ annular nebula, Lyra. . = q \-^ „ „ 



„ Dumb-bell nebula. . . . = 19 ^ 0l „ „ 



These values are too small by the unknown corrections for the possible 

 power of extinction of space, and for the absorptive power of the earth's 

 atmosphere. — Proceedings of the Royal Society, vol. xv. p. 18. 



* Philosophical Transactions, 1864, p. 154 and plate 1. 



For the purpose of ascertaining whether the absence of the other bright 

 lines of nitrogen might be connected with the presence of hydrogen, I ar- 

 ranged an apparatus in which, while the spectrum of the induction-spark in 

 a current of nitrogen was being observed, a current of hydrogen could be 

 introduced, and the proportion of the two gases to each other easily regu- 

 lated. With this apparatus the fading out of the bright lines of nitrogen, 

 as the proportion of this gas to hydrogen was diminished, and again their 

 increase in brilliancy when the current of nitrogen was made stronger, were 

 carefully observed, but without detecting any marked variation in the re- 

 lative brightness of the lines. 



f Philosophical Transactions, 1864, pp. 143, 150. 



