196 PROGRESS OF SCIENCE IN THE CENTURY. 



improved telescopes, such as that constructed by Lord 

 Rosse at Parsonstown in Ireland, resolved one nebula 

 after another into collections of stars. Indeed imag- 

 ination far outstripped the evidence, and it was wide- 

 ly supposed that nebulae were systems of suns, multi- 

 ples, as it were, of the architectural unit which our 

 solar system was believed to display. 



So far telescopic analysis had alone been possi- 

 ble, but the next great step was taken in 1864, 

 when Sir William Huggins applied the spectroscope 

 to the study of a small but bright nebulae in the con- 

 stellation of the Dragon. The spectrum (yielding 

 no continuous band) was like that of a glowing gas, 

 and therefore it was concluded that this nebula? was 

 not a galaxy of stars, but a vast area of incandescent 

 gas. In the next few years many others, including 

 the Great Nebulae of Orion, were shown to be gaseous 

 while others (yielding "continuous" spectra) seemed 

 to be either star clusters or gases in process of con- 

 densation. 



It is important to notice that the growth of ther- 

 modynamics has led to a rejection of the old view 

 that nebulous stuff was originally or is still " instinct 

 with fire." The essay of Helmholtz in 1854 made it 

 plain that this supposition is unnecessary, " since in 

 the mutual gravitation of widely separated matter we 

 have a store of potential energy sufficient to gener- 

 ate the high temperature of the sun and stars. We 

 can scarcely go wrong in attributing the light of the 

 nebulae to the conversion of the gravitational energy 

 of shrinkage into molecular motion.'' * 



" It is difficult not to see in the gaseous nebulae 

 the stuff of which future stars will be made. Grant- 

 ing that their substance is subject to the law of gmvir 

 * Huggins. Rep, Brit. Ass. for 1891, p. 22. 



