304 POPULAR SCIENCE MONTHLY. 



atmosphere surounding the arc. It is not difficult to see the bearing 

 of this discovery upon astrophysical inquiry. Some subjects are made 

 more complex ; but the hope is held out that eventually we may detect 

 these indications of pressure, differentially, in the brighter stars. 



It is also known that the spectra of some elements are altered by 

 the presence of other elements, but the extent and character of the 

 induced changes are little understood. As the chemical elements are 

 never found alone in celestial bodies, the serious consequences of this 

 effect must be evident. 



The temperature in glowing Pliicker tubes is of great interest, from 

 its bearing upon the probable temperatures of nebula?, the aurorse and 

 other bright-line phenomena of a diffuse nature. It is not certain 

 that direct observation by any thermometric device can deal with the 

 problem. The measures thus far attempted have assigned tempera- 

 tures but a few degrees higher than that of the environment. These 

 indications are probably correct for the average temperature of the 

 contents of the tube, but hardly so for those molecules which are 

 glowing. It has been suggested that perhaps a very small proportion 

 of the molecules receive and carry the discharge; that while the 

 molecules in action may be very hot, the average for all in the tube 

 is very low. It seems reasonable to suppose, also, that the low-tem- 

 perature indication is due to the fact that the current is actually 

 passing but a small fraction of the time. The effect upon the eye 

 is that of a continuous glow, whereas the thermometer measures the 

 average effect. 



The influence of a magnetic field upon the character of spectral 

 lines, established in the laboratory by Zeeman, has not yet been ob- 

 served in celestial spectra, but its detection may be merely a question 

 of the dispersive power available on faint spectra. 



It will be perceived that the interpretation of celestial spectra must 

 be made with circumspection. We are not always justified in reaching 

 conclusions upon the spectroscopic evidence alone; general conditions 

 must also be taken into account. For example, shall we say that the 

 temperature of the gaseous nebulae is very high, because they have 

 bright-line spectra? On the contrary, the difficulty of maintaining a 

 high temperature in a mass so attenuated should be given at least equal 

 weight. The radiating molecules or particles may for the instant be 

 quite hot, but the effective temperature of the whole nebula is prob- 

 ably low. 



The experimental verification of radiation pressure by Lebedew, 

 and by Nichols and Hull, is far-reaching in its consequences. We 

 must take this force into account, as truly and as constantly as we must 

 consider gravitation. Radiation pressure requires us to reconstruct 

 our theories of comets' tails, of the corona, of the zodiacal light, of 



