326 Prof. F. Zollner on the Temperature and 



mind the very small power of emission which permanent gases ex- 

 hibit compared with that of vaporized solid bodies. If we consider 

 the power of emission of very small quantities of different gases 

 at the same temperature for rays of the same refrangibility*, 

 we find that the above-mentioned experiment of Wullner affords 

 a good illustration of the extraordinary difference in the power 

 of emission, and therefore, according to KirchhofPs law, the power 

 of absorption of different gases at the same temperature. In 

 this experiment the small quantity of the sodium-vapour which 

 is volatilized in the Geissler's tube gives out more light than 

 hydrogen compressed under a pressure of 1000 millims. of mer- 

 cury. It is only by bearing in mind this circumstance that we* 

 can explain why, if on this ground the lines of nitrogen and 

 oxygen are absent, we yet see those of other elements whose 

 atomic weights and therefore densities are much greater than 

 those of oxygen and nitrogen. From these considerations the 

 following conclusions may either be immediately drawn, or may 

 be shown to follow by a train of reasoning which I propose more 

 fully to develope elsewhere. 



1. From the absence of certain lines in the spectrum of a self-* 

 luminous star the absence of the corresponding elementary body 

 cannot be affirmed. 



2. The absorbent layer in which the reversal of the spectrum 

 occurs is different for each element. This point lies nearer to 

 the centre of the star the greater the density and the smaller 

 the power of emission of the element in question. 



3. In the case of different stars this layer lies, under otherwise 

 similar circumstances, nearer to the centre the greater the inten- 

 sity of gravitation is. 



4. The separation of the layers of reversion of the several 

 elements, both from one another and from the centre of the star, 

 increase with rise of temperature. 



5. The spectra of different stars are, under otherwise similar 

 circumstances, richer in lines the lower the temperature is and 

 the greater the mass of the star. 



6. The great difference in intensity exhibited in the lines of the 

 solar spectrum and the spectra of other fixed stars not only de- 

 pends upon the differences in the power of absorption, but also 

 upon the different depths of the layers in which the reversal of 

 the several spectra occurs. 



In conclusion, I may be allowed to make some remarks on the 

 application to the heavenly bodies of the results of experiments 

 made upon rarefied gases. It has lately been suggested by Lecoq 



* A perfect transparency of the mass of gas for the emitted ray is here 

 assumed — an assumption which more nearly approaches the truth as the 

 masses of the gases under comparison become smaller. 



