On  some  recent  Researches  in  Solar  Physics.  385 
Argentic  fluoride  did  not  dissolve  in  bisulphide  of  carbon.  By- 
passing the  vapour  of  the  latter  substance  over  the  silver-salt  at 
a  red  heat,  a  chemical  change  took  place,  and  a  fuming  acid  vapour 
was  evolved,  in  accordance  with  the  following  equation  : — 
4AgF+CS2=2Ag2S  +  CF4. 
A  solution  of  bromine  or  iodine  in  bisulphide  of  carbon  was  rapidly 
decolorized  by  agitation  with  particles  of  argentic  fluoride,  and  the 
liquid  acquired  the  odour  of  tetrafluoride  of  carbon. 
December  21. — George  Biddell  Airy,  C.B.,  President,  in  the  Chair. 
The  following  communications  were  read  : — 
M  On  some  recent  Researches  in  Solar  Physics,  and  a  law  regula- 
ting the  time  of  duration  of  the  Sun-spot  Period."  By  Warren  De 
La  Rue,  D.C.L.,  F.R.S.,  Balfour  Stewart,  F.R.S.,  and  Benjamin 
Loewv,  F.R.A.S. 
1.  In  the  short  account  of  some  recent  investigations  by  Professor 
Wolf  and  M.  Fritz  on  sun-spot  phenomena,  which  has  been  published 
lately  in  the  'Proceedings  of  the  Royal  Society'  (1871,  vol.  xix. 
p.  392),  it  was  pointed  out  that  some  of  Wolf's  conclusions  were 
not  quite  borne  out  by  the  results  which  we  have  given  in  our  last 
paper  on  Solar  Physics  in  the  Philosophical  Transactions  for  1870, 
pp.  389-496.  A  closer  inquiry  into  the  cause  of  this  discrepancy 
has  led  us  to  what  appears  a  definite  law,  connecting  numerically 
the  two  branches  of  the  periodic  sun-spot  curve,  viz.  the  time  du- 
ring which  there  is  a  regular  diminution  of  spot-production,  and  the 
time  during  which  there  is  a  constant  increase. 
It  will  be  well,  for  the  sake  of  clearness,  to  allude  here  again,  as 
briefly  as  possible,  to  Professor  Wolf's  results  before  stating  those 
at  which  we  have  arrived. 
2.  Professor  Wolf  had  previously  devoted  the  greater  part  of  his 
laborious  researches  to  a  precise  determination  of  the  mean  length  of 
the  whole  sun-spot  period,  but  latterly  he  has  justly  recognized  the 
importance  of  obtaining  some  knowledge  of  the  average  character  of 
the  periodic  increase  and  decrease.  Hence  he  has,  as  far  as  he  has 
been  able  to  do  so  by  existing  series  of  observations,  and  his  peculiar 
and  ingenious  method  of  rendering  observations  made  at  different 
times  and  by  different  observers  comparable  with  each  other,  endea- 
voured to  investigate  more  closely  the  nature  of  the  periodic  sun- 
spot  curve  by  tabulating  and  graphically  representing  the  monthly- 
means  taken  during  two  and  a  half  years  before  and  after  the 
minimum,  and  applying  this  method  to  five  distinct  minimum 
epochs,  which  he  has  fixed  for  the  following  years : — 
1823*2 
1833-8 
1844-0 
1856-2 
1867-2 
Phil  Mag.  S.  4.  Vol.  43.  No.  287.  May  1872.         2  C 
