The Nehdar Hijijothesis : Its Present Condition. 175 



all circumstances, it seemed very probable indeed that indefinitely in- 

 creased telescopic power was all that was needed to resolve the remain- 

 der. Still it retained a few believers, loth to relinquish the insight 

 into the origin of the solar system which it holds out, and ready to 

 sieze at once on the important discovery of true gaseous nebulae by Mr. 

 Huggins, as direct confirmation of the hypothesis. There can be no 

 question that the proved existence of immense masses of gaseous mat- 

 ter does place the theory upon a much firmer footing than before ; at 

 least, the speculations of Sir William Herschel are now established, 

 notwithstanding that the superstructure raised thereon by Kant and 

 Laplace is little, if, indeed, as we shall see further, at all supported 

 by it. 



For this important advance in our knowledge \ve are indebted to the 

 spectroscope. When all other means had failed to give us fiiithful in- 

 dications of the existence of remote gaseous bodies, this invaluable in- 

 strument at once decided without question that, of the nebulae unre- 

 solved by the largest and most powerful telescopes, a considerable pro- 

 portion are simply mechanical mixtures of three gases, tAvo of which 

 are well known to us, namely, nitrogen and hydrogen ; and the third, 

 although unknown or unidentified, is probably of somewhat similar 

 character — that is, an elementary, nonliquefiablegas. It is not improb- 

 able that it may even be a terrestrial element, for our knowledge of 

 the variety of spectra obtainable from the same substance, under dif- 

 ferent circumstances, is still too deficient to enable us to speak positive- 

 ly on this point. But the spectroscope is capable of telling us still 

 more regarding the nature of the light analyzed by it, and it is here 

 that the evidence it gives is unfavorable to the hypotheses of Laplace. 

 The density of the gas from which the light emanates produces an 

 effect upon the spectrum, and is measured by the breadth of the lines 

 composing it. Now, the nebular hypothesis requires as a necessary 

 corollary — and it has accordingly always been admitted as such — that 

 nebulae of every degree of condensation should be found in the heav- 

 ens, and the variations of brilliancy of these bodies has therefore been 

 pointed out as evidence of variation of density. The width of the spec- 

 tral lines, however, provides us with a much more certain, delicate, 

 and reliable test. 



From the observations of Dr. Huggins, it would appear that the 

 bright lines in the nebular spectra present no appreciable difference 

 of thickness in all those cases in which it has been possible to use a 

 very narrow slit. The lines have invariably been found to be exceed- 

 ingly fine ; and hence we are furnished with distinct proof that the 

 gases so examined are not only of equal or nearly equal density, but 



