Amstpt.?il>92arm-}   Analysis  of  Coal  Tar  Preparations.  485 
parison  of  tar  oil  preparations,  owing  to  the  element  of  uncertainty 
that  enters  into  individual  experiments  and  the  great  variations  in 
the  results  occasioned  by  very  slight  deviations  in  the  experimental 
conditions,  such  as  are  certain  to  obtain  in  the  hands  of  different 
investigators.  As  the  germicidal  values  of  the  principal  chemical 
constituents  of  the  various  tar  preparations  in  commerce  are  known, 
it  appears  indeed  preferable  to  abandon  direct  bacteriological  inves- 
tigation and  define  the  composition  of  the  product  from  a  chemical 
point  of  view,  from  which  its  true  value  as  a  disinfectant  can  be 
easily  and  reliably  estimated. 
Composition  of  Tar  Oils.  — The  tar  oils,  whether  distilled  from  coal, 
or  wood,  or  bones,  are,  however,  of  such  a  complicated  character 
that  the  task  of  separating  them  and  estimating  each  individual  con- 
stituent would  be  not  only  formidable  but  absolutely  impracticable 
in  commercial  analysis.  Nor  is  such  a  detailed  investigation  at  all 
necessary  to  arrive  at  an  approximate  and  reliable  valuation  of  tar 
oils  or  preparations  made  from  tar  oils  as  disinfectants.  Roughly 
speaking,  the  constituents  of  tar  oils  may  be  classified  into  three 
divisions:  (1)  The  Tar  Acids.  (2)  The  Bases.  (3)  The  Hydro- 
carbons, according  to  whether  their  acidic  or  basic  nature  most  pre- 
dominates, or  whether  they  are  of  an  entirely  neutral  character  like 
hydrocarbons. 
In  the  tar  oils  distilled  from  coal,  which,  of  the  different  tar  pro- 
ducts, are  generally  or  indeed  almost  without  exception  employed 
in  the  crude  state  in  the  manufacture  of  disinfectants,  the  hydrocar- 
bon constituents  form  by  far  the  larger  proportion,  and  even  these 
vary  in  chemical  character  and  disinfectant  value  according  to  the 
origin  and  nature  of  the  coal,  the  mode  of  distillation,  the  regula- 
tion of  the  temperature  of  distillation,  and  numerous  minor  details 
that  more  or  less  affect  the  chemical  changes  occurring  during  dis- 
tillation and  the  composition  of  the  resulting  products.  Amongst 
the  ordinary  commercial  coal  tar  oils  three  products  are  especially 
distinguished  as 
Light  Oil,  the  fraction  below  1500  C,  with  a  specific  gravity  of 
09,  and  consisting  principally  of  benzene  and  its  homologues,  and 
some  naphtalene ; 
Medium  Oil,  distilling  between  1500  C.  and  2100  C,  possessing 
a  specific  gravity  of  about  I  01,  and  containing  principally  naphta- 
lene and  other  hydrocarbons,  carbolic  acid,  cresols  and  bases ;  and 
