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 Plücker tubes is of great interest, from its bearing upon the probable temperatures of nebulæ, the auroræ 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 temperatures 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-temperature 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 observed 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 nebulæ 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 probably 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
