728  Intelligence  and  Miscellaneous  Articles. 
To  sum  up,  the  measurements  described  confirm  the 
existence  of  a  retardation  o£  the  a  rays  when  they  traverse  a 
sheet  of  aluminium  such  as  has  been  observed  by  Rutherford. 
The  a.  rays  of  radium  have  behaved  in  these  experiments 
like  the  a  rays  of  bodies  rendered  active  by  the  emanation. 
I  have  been  obligingly  assisted  in  the  above  experiments 
by  M.  Matout,  preparation-room  assistant  at  the  Museum. 
LXV.  Notices  respecting  New  Books. 
Ueber  den  Gegenwdrtigen  Stand  der  Frage  nach  einer  Mechanischen 
Erklarung   der  Elektrischen   Erscheinungen.      Von  Hans  Witte, 
Dr.  jphil.   Mit  14  Eiguren  und  einer  Taf el.    Berlin :  E.  Ebering. 
1906.     Pp.  xii  +  232. 
T^EW  aspects  of  physical  science  have  claimed  the  attention  of  so 
-*-  many  distinguished  investigators  as  the  dynamical  inter- 
pretation of  electromagnetic  phenomena.  The  number  of 
different  dynamical  theories  of  the  electromagnetic  field  is  some- 
what perplexing,  and  the  student  who  first  approaches  the  subject 
cannot  help  feeling  bewildered  by  the  enormous  mass  of  material 
to  be  dealt  with.  The  author  of  the  mouograph  under  review, 
which  is  the  first  of  a  series  to  be  issued  under  the  general  title  of 
yatunuissenschaftliche  Stitdien,  has  performed  a  signal  service  by 
his  remarkably  lucid  and  thoroughly  systematic  examination  of 
the  various  possible  dynamical  theories  of  electromagnetic  pheno- 
mena. In  it  he  has  included  all  past  and  present  theories,  and  has 
clearly  exhibited  their  relationships.  The  work  is  divided  into 
4  sections.  Section  I.  deals  with  general  notions,  fundamental  laws 
and  classification  of  theories.  The  very  brief  Sections  II.  and  III. 
are  devoted  to  action-at-a-distance  and  emission  theories  respec- 
tively— now  only  of  historical  interest.  Section  IV.,  which  forms 
the  bulk  of  the  book,  contains  an  examination  of  wave  theories. 
A  useful  bibliography  is  given  at  the  end  of  the  book.  We  can 
heartily  recommend  this  unique  monograph  to  all  advanced  students 
of  electromagnetic  theory. 
LXVI.  Intelligence  and,  Miscellaneous  Articles. 
To  the  Editors  of  the  Philosophical  Magazine. 
Gentlemen,—  24th  April,  1906. 
SINCE  the  publication  of  my  note  on  a  mercury-sealed  tap  in 
your  March  number  I  have  received  from  Dr.  Thiele,  of 
Dresden,  a  copy  of  a  paper  published  in  the  Annalen  der  PhysiJc  in 
,1901,  Bd.  6,  in  which  he  describes  a  tap  of  design  identical  with 
mine,  as  well  as  certain  improvements  upon  it. 
I  much  regret  that  I  was  not  previously  acquainted  with 
Dr.  Thiele's  work,  and  I  shall  esteem  it  a  favour  if  you  will  insert 
this  letter  in  your  Magazine. 
I  am,  Gentlemen, 
Tours  obediently, 
A.  P.  Chattock. 
