We  find  therefore  that  under  conditions  of  pressure  this  diffusion  is  retarded  and
the  cross-section  or  unit  space  tends  to  be  more  constant  or  protracted,  there  being
a  diminution  of  radiation.  In  other  words,  where  conditions  of  high  pressure
delivery  maintain,  there  is  a  resultant  cohesion  or  condensing,  due  to  the  initial
pressure,  which  tends  to  retain  the  combustion  within  a  more  confined  flame  area
subtending  a  decrease  of  radiation.
Velocity  of  Flame  Propagation. — Again,  another  feature  with  which  we  are  not
exactly  acquainted,  comes  with  the  fact  that  the  compression  of  the  gas  brings  its
molecules  or  atoms  into  closer  juxtaposition,  and  the  transmission  of  heat  evolved
is  more  rapid  and  complete.  This  is  shown  by  actual  tests,  which  go  to  prove  that
under  conditions  of  high  pressure  deliverj',  the  same  amount  of  heat  is  evolved  with
less  fuel,  or  greater  heat  with  an  equal  fuel  under  combustion.  We  might  term  this,
through  lack  of  a  better  word,  "  heat  propagation,"  as  the  action  is  analogous  to
that  of  flame  propagation,  which  latter  is  undoubtedly  a  factor  in  the  radiation
activity  herein  described.
Another  manifestation  of  this  heat  propagation,  or  more  strictly  speaking,  propa-
gation of  temperature,  is  seen  in  the  cylinder  of  the  gas  engine  under  high  compression.
This  is  possibh'  the  best  illustration  that  we  have  of  the  conditions  of  combustion
due  to  a  compressed  gas,  although,  in  addition  to  the  "  radio-activity  "  which  we
have  just  mentioned,  there  is  in  this  condition  the  added  value  of  the  fuel,  due  to  the
compression  of  a  much  larger  amount  of  combustible  within  a  given  space,  in  fact
doubling  the  amount  of  this  combustible  at  the  pressure  of  each  additional  atmosphere.
