86 
VARIETIES. 
the  most  part  of  ethereal  oils,  and  contains  only  a  small  per-centage  of 
creosote  or  of  carbolic  acid.  He  recommends,  for  the  examination  of  this 
oil,  mixing  a  known  volume  with  10  per  cent,  of  strong  caustic  alkaline 
solution  in  a  graduated  tube,  and  shaking  the  mixture  well.  After  a  time 
it  separates  into  three  layers,  the  lower  one  being  pure  caustic  alkali,  the 
middle  one  contains  the  creosote  and  carbolic  acid,  and  the  upper  layer 
consists  of  the  ethereal  oils.  As  the  real  value  of  the  oil  used  for  impreg- 
nating wood  depends  upon  the  amount  of  creosote  and  carbolic  acid  it  con- 
tains, this  method  may  serve  for  its  valuation.  In  this  way  Dr.  Vohl  has 
found  that  coal  tar  oil  obtained  from  England,  France  and  Belgium,  does 
not  contain  at  the  most  more  than  8  or  10  per  cent,  of  creosote  and  carbolic 
acid,  while  the  oil  obtained  in  the  production  of  photogen  contains  at  least 
70  per  cent,  of  these  subtances. 
The  impregnation  of  ships'  cordage  and  sail-cloth  with  creosote  is  effected 
by  means  of  the  combination  of  creosote  with  gelatinous  substances,  such 
as  skin,  leather,  &c.  For  this  purpose  the  cloth  or  rope  is  immersed  in  a 
dilute  solution  of  gelatine,  and  then  passed  through  a  bath  of  oak-bark 
decoction,  after  which  it  is  impregnated  with  creosote.  Sail-cloth  treated 
in  this  way  has  been  in  ordinary  use  for  six  years  without  showing  signs 
of  decay. — Lond.  Pharm.  Jour.,  Oct.  1857. 
Method  of  Cleaning  Soiled  Silver  Vessels,  &c  — Prof.  Bottger  states  that 
silver  utensils,  which  are  stained  by  sulphuretted  hydrogen,  &c,  may  be 
easily  cleaned,  by  immersing  them  in  a  boiling  saturated  solution  of  borax, 
or  a  moderately  concentrated  solution  of  caustic  potash  in  contact  with 
metallic  zinc.    A  zinc  sieve  may  be  used. — Ibid. 
Theory  of  Substitution. — The  theory  of  substitutions  has  been  attributea 
by  some  to  Dumas,  and  by  others  to  Laurent,  and  others  still  to  Gay 
Lussac.  The  question  of  priority,  discussed  by  Laurent  in  his  Methode  de 
Chimie,  p.  241,  has  been  recently  taken  up  by  Dumas,  who  has  established 
the  precise  facts  upon  exact  documentary  evidence.  It  appears  that  the 
first  idea  of  substitution  is  due  to  Dumas,  who,  on  the  13th  of  January, 
1834,  made  the  formal  statement  that,  "  when  a  hydrogenated  substance  is 
subjected  to  the  action  of  any  dehydrogenating  substance,  it  takes  up  a  portio  n 
of  the  latter,  equivalent  to  that  of  the  hydrogen  lost."  Dumas  at  this  same 
time  designated  this  class  of  phenomena  by  the  word  metalepsy,  an  expres- 
sion which  has  been  attributed  to  Berzelius. 
But  the  theory  of  substitutions  required  for  its  completion  a  correct 
knowledge  of  the  part  in  the  changes  played  by  chlorine.  It  will  be  remem- 
bered that  the  idea  of  Laurent  that  "  in  the  bodies  obtained  by  substitution, 
chlorine  not  only  takes  the  place  of  hydrogen,  but  acts  the  same  part  with 
it,"  met  with  general  denunciation.  It  was  followed  by  the  bitter  criticisms 
of  Berzelius,  Liebig,  and  Wohler,  and  a  declaration  by  Dumas  disclaiming 
