euth  Rnd  tbe  otlier  planeta,  is  even  moro  oKtural,  and  hMd
been  drawn  by  several  eminent  aBtronomcrs.  j

It  ii  further  quit^i  immaterial  to  the  tlieory  of  a  homoa
geneoua  universe  bow  dense  the  several  bodies  of  an;  system
may  be.  In  the  solar  system  we  find  the  densest  planet
(Mereary)  sixteen  times  denser  than  the  least  dense  one
{,&atnm),  while  the  Earth  is  four  and  Mercury  eight  titiiea
as  dense  as  the  Sun.
Diminution  of  density  requires  a  proportional  ipcreastf

of  volume,  and  we  may  conceive  a  body  attenuated  into  C

ntere  gu,  and  occupying  a  vast  amount  of  apace.     An  entire

•jBtem  may  thus  be  imagined  to  consist  of  a  single  body  in

'^ii  rarefied  state.     Such,  in  fact,  arc  the  true  neiul<e,  and

^      those  already  discovered  and  tested  by  the  spectrosco[ie  pre-

Wntnumeroiia gradations  from  this  ideal  initial  forni  tostates

*hich  exhibit  marked  tracea  of  organization  and  internal

'ufferentiation.     We  have  the  spiral  nebula  in  Cttnei  Veni^

'^oa,  with  evidence  of  the  development  of  now  centera  near

'ta  oater  limit.     We  hare  also  the  annular  nebula  in  /.t/ra

^^th  a  very  definite  organization,  and  we  have  many  other

,'^»narkablc   forms   which   indicate   tliat  these   nebulae  are

**%    reality  nosoent  systems.     Moreover,  certain  star-systenia

•«*.<)w  marks  of  more  or  less  imperfection  in  their  dcvclop-

^^«nt,and  some  of  the  fixed  stars  themselves  afford  an  unceiv

^^dn  and  partially  nebular  spectrum.
