THEORY OF ETHER 169 



collision of particles probably brings about all kinds of vibra- 

 tions, and hence all kinds of ether- waves, of which many are 

 no doubt invisible. The particles of a gas, unless at a high 

 pressure, suffer fewer collisions, and hence give out a few 

 characteristic vibrations, forming distinct line spectra. Our 

 main source of light, the sun, emits waves of all lengths, not 

 only white light and heat, but waves which do not affect our 

 senses, some of them having been made apparent by special 

 means. This is what we should expect from such a body as 

 the sun. The special line spectra from which so much may be 

 learnt as to the composition of bodies must be studied cau- 

 tiously. We shall find that the spectrum characteristic of a 

 given body at a given temperature is not necessarily the same 

 as its spectrum at another temperature, or as the one yielded 

 by an electric discharge passing through it. Also, a change of 

 pressure may bring about modifications. Although conclusions 

 must not be drawn too freely from spectroscopic observations, 

 since a given vibration may be due to several others com- 

 bining, or may be modified by neighbouring particles, yet 

 they have become a most important means of analysis, far ex- 

 ceeding in delicacy any other method. New conceptions also 

 as to what is truly elementary matter have been gained from 

 spectroscopic investigations. 



Another important branch of spectroscopic work is the 

 investigation of the results of transmitting ordinary light 

 through incandescent vapours. We find under certain con- 

 ditions that the usual characteristic bright lines are replaced 

 by dark spaces. In other words, the characteristic vibrations 

 previously emitted are now absorbed. The particles of the 

 vapour select and check those vibrations which are, as it were, 

 attuned to their own periods. It is obvious if this result is 

 to be obtained that the vapour must not be at a temperature 

 high enough to yield of itself a bright spectrum. The absorp- 

 tion lines are conveniently seen in the spectrum of the sun, 

 which shows numerous dark spaces corresponding with the 

 bright lines emitted by known terrestrial vapours. A colder 

 envelope of vapours of very complex composition sifts various 

 portions of the otherwise continuous spectrum of the sun. A 



