364  Germs  and  Disease.  {AmjSy,tiSarm- 
foundation  for  the  methods  now  adopted  by  Pasteur  for  the  cure  of 
hydrophobia  and  by  Koch  for  the  cure  of  consumption,  methods 
which  are  now  riveting  the  attention  of  the  whole  civilized 
world. 
In  consideration  of  the  intense  interest  now  being  taken  in  the 
work  going  on  in  Berlin  and  the  centres  supplied  from  it,  some 
explanation  of  the  methods  adopted  for  proving  the  presence  of  a 
disease-producing  organism  and  for  checking  its  ravages  may  not,  it 
is  hoped,  be  devoid  of  interest.  The  chain  of  evidence  on  which 
bacteriologists  rely  as  proof  of  the  presence  of  disease-producing 
bacteria  is  as  follows  : 
(1)  The  organism  must  be  found  (by  the  aid  of  the  microscope) 
in  the  blood  or  tissues  of  the  affected  animal. 
(2)  It  must  be  transferred  from  these  to  nutrient  solutions  con- 
tained in  test  tubes  (the  nutrient  materials  of  these  solutions  consist 
of  gelatin  and  other  meat  products)  and  cultivated  therein.  These 
cultivations  must  be  carried  on  through  successive  generations  of 
the  organism. 
(3)  A  "  pure  cultivation  "  obtained  in  this  way  must,  when  intro- 
duced into  a  healthy  animal,  produce  the  disease  in  question. 
-  (4)  In  the  inoculated  animal  the  same  micro-organism  must  again 
be  found. 
In  forging  a  chain  of  evidence  such  as  this  is,  the  greatest  skill  is 
required  and  the  utmost  precautions  must  be  taken  to  prevent  the 
entrance  into  the  pure  cultivations  of  other  bacteria,  which  are 
present  almost  everywhere.  All  the  apparatus  used  must  be 
"  sterilized,"  and  the  slightest  neglect  of  details  like  this  leads  to 
hopeless  confusion.  Bacteriology  is,  in  fact,  an  art  requiring  of  its 
workers  exceptional  qualities  and  special  training. 
The  methods  adopted  for  curing  or  preventing  diseases,  which 
owe  their  origin  to  bacteria,  vary  with  the  disease  under  treatment, 
and  the  principles  underlying  them  are  not  yet  removed  from  the 
domain  of  theory.  Pasteur,  in  his  famous  researches  on  hydro- 
phobia, found  that  by  various  methods  he  could  attenuate  the  virus 
of  rabies  so  that  when  injected  into  a  healthy  animal  it  not  only 
did  not  produce  any  untoward  symptoms,  but  it  actually  prevented 
the  most  active  virus  from  having  any  effect.  The  method  he  now 
adopts  is  that  of  suspending  pieces  of  the  spinal  cord  of  inoculated 
rabbits  in  bottles  of  air,  dried  by  means  of  potash.    After  hanging 
