THEOPENSPACEINORION^245"Thenebulais aluminousbody,butordinarygas
isinvisible. Wedonotseethegaseswhichsurround
us,andwhichformtheatmosphereinwhichwelive.
How,then,ifthenebulaconsistedmerelyofgaseous
matter,wouldweseeitshiningonthefar-distantheav-
ens? Awell-knownexperimentmaysufficetoremove
thisdifficulty. Wetakeatubecontainingaverysmall
quantityofsomegas: forexample,hydrogen, which
isusually invisible:no onecouldtell thattherewas
anygasinthetube,orstilllesscoulditscharacterbe
known;butpoura streamofelectricitythrough the
tube,andinstantlytheinteriorbeginstoglow witha
violetlight. Whathastheelectricitydoneinthisex-
periment? Itssoleeffecthasbeentoheatthegas. It
is,indeed,merelyaconvenientmeansofheatingthegas
andmakingitglow. Wedonotseetheelectricity,it
is ratherthegasheatedbytheelectricity. Weinfer,
then,thatifthegas beheateditbecomes luminous.
Itdoesnotburnintheordinarysenseoftheword;no
chemical changetakesplace. Thetube containsex-
actlythesameamountofhydrogen afterthe experi-
mentthatitdidbefore. Itglows with heatjustas
red-hotironglows. If,then,wecouldbelievethatin
thegreatnebulainOriontherewerevastvolumesof
rarefiedgasinthesamephysicalconditionasthegas
inthetubewhiletheelectricitywaspassing,thenwe
shouldexpecttofindthatthisgaswouldactuallyglow.
'Tosettlethequestionas totherealnatureofthe
nebulae,wemustcallintheaidofthatrefinedmethod
ofinvestigationknownasspectrumanalysis.. ..The
spectrumofa gaseousnebulais a remarkable sight.