TECHNICAL  ANALYSIS  OF  SOAP. 
853 
It  was  only  with  the  sulphurous  acid  that  any  change  was 
noticed.  The  tubes  containing  this  substance  remained  clear 
during  tivo  months^  but  after  that  time  a  disturbance  set  in 
which  slowly  increased,  and  sulphur  was  deposited  in  a  finely 
divided  state. 
The  sulphurous  acid  was  thus  gradually  reduced  to  sulphur, 
but  the  oxygen  was  not  liberated,  another  part  of  the  acid  hav- 
ing been  oxydized  by  it  to  sulphuric  acid.  It  seems  very  sin- 
gular that  a  space  of  two  months  elapsed  before  any  change  was 
observed ;  it  appears  that  the  absorption  of  a  great  amount  of 
light  was  necessary  for  the  separation  of  the  first  atom  of  sul- 
phur, which  was  followed  then  by  more  atoms  in  much  shorter 
intervals  of  time. — Amer.  Jour.  Science  and  Arts, 
New  York,  December,  1869. 
ON  THE  TECHNICAL  ANALYSTS  OF  SOAP  * 
By  M.  Gaston  Tissandier. 
The  name  of  soap  is  given  to  true  salts,  formed  by  combining 
fatty  acids  (oleic,  margaric,)  with  alkalies,  such  as  soda  or  pot- 
ash. The  quality  of  a  soap  is  ascertained  by  determining  the 
proportion  of  fatty  acid  and  alkali  which  it  contains,  and  also 
the  foreign  substances — such  as  chlorides,  alkaline  sulphates, 
moisture,  &c. — which  always  occur  in  varying  proportions. 
Fatt^  Acids. — Dissolve  5  grms.  of  the  soap  in  question  in  J  a 
litre  of  distilled  water,  heated  in  a  porcelain  capsule ;  when  dis- 
solved, add  a  slight  excess  of  dilute  sulphuric  acid,  and  let  it 
boil  for  some  minutes,  so  that  the  fatty  acids  may  become  sepa- 
rated and  float  upon  the  liquid.  To  weigh  the  fatty  acids,  cool 
them,  and  they  will  form  a  cake  of  grease,  which  must  then  be 
fused,  in  order  to  dry  them,  in  a  small  tared  porcelain  capsule ; 
this  capsule,  when  again  weighed,  will  give  the  amount  of  fatty 
acids  corresponding  to  5  grms.  of  soap. 
Wax  may  also  be  used  to  facilitate  the  weighing.  After  the 
first  part  of  the  operation  has  been  performed,  and  the  fatty 
acids  are  floating,  add  7  grms.  of  white  wax,  which  will  melt 
*  Moniteur  Scientifique. 
23 
