Notices  respecting  New  Books.  149 
It  may  be  remarked,  in  the  third  place,  that  the  same  objec- 
tion, namely  that  two  particles,  which  begin  with  finite  velocity, 
attain  infinite  vis  viva  at  a  finite  distance  from  each  other,  applies 
also  to  the  law  of  gravitation,  if  it  is  assumed  that  the  masses 
of  ponderable  particles  are  concentrated  in  points.  But  if  this 
objection  is  got  rid  of,  in  the  case  of  the  law  of  gravitation,  by 
assuming  that  the  masses  even  of  the  smallest  particles  occupy 
space,  we  must  make  the  same  assumption  in  relation  to  elec- 
trical particles,  in  which  case  it  results  that  only  a  vanishingly 
small  part  of  such  a  particle  arrives  at  a  given  instant  at  the 
distance  p  ;  another  vanishingly  small  part,  which  arrived  at 
the  distance  p  at  the  previous  instant,  will  have  exchanged  its 
infinitely  great  velocity  of  approach  for  an  infinitely  great  velocity 
of  separation.  But  if  these  vanishing  parts  of  the  smallest  par- 
ticles are  solidly  connected  together,  there  cannot  be  any 
question  of  such  infinite  velocities  at  all. 
Even  cosinical  masses  may  begin  their  movements  under 
physically  admissible  conditions,  and,  by  continuing  to  move 
according  to  the  law  of  gravitation,  may  come  into  physically 
inadmissible  conditions,  which  can  be  avoided  only  through  the 
cooperation  of  molecular  forces  confined  to  molecular  distances. 
The  disregard  of  this  cooperation  is,  strictly  speaking,  only 
temporarily  allowable,  namely  so  long  as  the  conditions  are 
such  that  its  influence  is  either  nothing  or  may  be  regarded  as 
vanishingly  small.  But  just  as  little  as  an  objection  to  the  law 
of  gravitation  is  derived  from  this  fact,  ought  any  objection  to 
the  fundamental  law  of  electrical  action  to  be  derived  from  the 
physically  inadmissible  conditions  to  which,  according  to 
Helmholtz,  this  lawr  leads,  when  it  is  considered  that  these  in- 
admissible conditions  are  connected  only  with  certain  molecular 
distances. 
XVI.  Notices  respecting  New  Books. 
Theory  of  Heat.  By  J.  Clerk  Maxwell,  M.A.,  F.R.S.,  Professor 
of  Experimental  Physics  in  the  University  of  Cambridge.  London  : 
Longmans,  Green,  and  Co.     1871.    (Pp.  312.) 
THE  subject  of  this  work  is  correctly  indicated  by  its  title  ;  it  is  a 
treatise  on  the  Theory  of  Heat;  its  writer's  aim  having  been  to 
state  and  enforce  the  general  propositions  that  have  been  established 
regarding  the  nature  and  effects  of  heat,  rather  than  to  discuss  the 
particular  facts  which  are  summed  up  in  those  propositions.  It 
must  not  he  supposed,  however,  that  no  notice  is  taken  of  the  expe- 
riments which  form  the  basis  of  our  knowledge  of  Heat;  on  the  con- 
trary, they  are  described,  where  necessary,  at  sufficient  length  to 
bring  out  the  principles  involved  in  them;  but  they  are  described 
