160  Intelligence  and  Miscellaneous  Articles. 
the  same  force  as  in  its  lengthening, -by  which  part  of  the  force 
which  would  be  exerted  in  contraction  is  consumed  in  internal  work 
and  changed  into  heat.  Such  an  internal  work  must  of  course  also 
take  place  in  stretching,  by  which  caoutchouc  becomes  heated — by  a 
series  of  extensions  and  contractions,  because  part  of  the  force 
exerted  in  extension  is  changed  into  heat,  and  also  because  part  of 
the  mechanical  force  which  would  be  developed  in  contraction  is 
also  changed  into  heat.  We  might  perhaps  say  that  caoutchouc  is 
like  a  pasty  mass  in  which  the  particles  rub  against  each  other,  as 
in  the  production  of  heat.  However  this  may  be,  the  explanation 
previously  given  of  the  development  of  heat  finds  a  brilliant  con- 
firmation in  experiments  which  Warburg  has  recently  made  in  the 
celebrated  laboratory  of  Magnus.  He  proves  by  various  experi- 
ments that  solids,  when  they  give  or  transmit  a  tone,  become 
heated,  because  the  original  vis  viva  is  changed  into  internal  work 
and  into  heat,  by  which  the  bodies  develope  more  heat  than  the 
sound-motion  inherent  in  them  or  transmitted  by  them  destroy. 
Of  all  bodies  investigated,  caoutchouc  is  that  which  becomes  most 
heated,  because  it  most  rapidly  extinguishes  the  sound- vibrations.  The 
analogy  between  Warburg's  observations  and  my  own  is  complete  ; 
and  the  explanations  agree  completely  with  the  data  previously 
given.  We  may  then  draw  the  conclusion  that  those  bodies  which 
become  cooled  on  stretching,  and  heated  on  contraction,  must  also  be 
heated  by  a  series  of  repeated  rapid  extensions,  because  in  such 
bodies  also  part  of  the  force  consumed  must  change  into  internal 
work  and  therefore  into  heat. — Poggendorff's  Annalen,  No.  10,  1871. 
ACTUAL  ENERGY. 
To  the  Editors  of  the  Philosophical  Magazine  and  Journal. 
Gentlemen, 
At  page  92  of  Professor  Clerk  Maxwell's  *  Theory  of  Heat '  (which 
I  have  no  hesitation  in  characterizing  as  the  best  existing  elementary 
treatise  on  any  branch  of  physical  science)  I  find  the  following  state- 
ment:— "We  cannot  even  assert  that  all  energy  must  be  either  po- 
tential or  kinetic,  though  we  may  not  be  able  to  conceive  any  other 
form." 
Now  I  have  to  remark  that  this  was  the  very  reason  which  induced 
me  in  1853  to  propose  the  word  "  actual "  for  denoting  energy  that 
is  not  potential,  rather  than  any  word  expressly  denoting  motion, 
and  which  still  induces  me  to  prefer  the  word  "  actual "  to  the  word 
"  kinetic  "  for  that  purpose. 
I  am,  Gentlemen, 
Your  most  obedient  Servant, 
W.  J.  Macquorn  Rankine. 
Glasgow,  January  23,  18/2. 
