SOLUBILITY  OF  GLUE  IN  GLYCERIN. 
517 
and  retained  its  hardness  in  damp  weather  ;  the  brown  glue  was 
4  to  5  times  thicker  than  the  former,  and  slightly  flexible. 
After  No,  1  had  been  standing  for  24  hours  without  examina- 
tion the  glue  was  found  to  be  still  firm ;  it  could  readily  be 
broken  into  smaller  pieces,  but  the  sharp  edges  observable  before 
were  now  softer  and  the  surface  of  the  glue,  after  removing  the 
adhering  glycerin  by  bibulous  paper,  was  soft  and  readily  scraped 
off.  It  became  very  evident  that  cold  glycerin,  undoubtedly 
owing  to  its  tenacity,  permeates  glue  very  slowly.  The  glycerin, 
however,  had  dissolved  a  notable  quantity  of  gelatine,  which  was 
readily  proven  by  the  flocculent  precipitate  produced  by  a  solu- 
tion of  tannic  acid. 
When  No.  2  had  been  digested  in  boiling  water  for  15  min- 
utes, the  glycerin  likewise  yielded  a  precipitate  with  tannin,  but 
digestion  for  half  an  hour  failed  to  unite  the  two  substances 
completely.  During  the  subsequent  maceration  for  about  a  week 
the  glue  continued  very  slowl.y  to  swell,  and  by  the  final  diges- 
tion for  one  hour  it  united  with  the  glycerin,  forming  on  cooling 
a  firm  jelly.  No.  3  was  occasionally  stirred,  and  finally  yielded 
an  elastic,  rather  soft  jelly.  No.  4  under  the  same  circumstances 
became  almost  gelatinous,  the  mass  flowing  about  the  same  as 
thick  Venice  turpentine.  No.  5  was  completely  dissolved  after 
digestion  for  15  minutes,  the  solution  gelatinizing  on  cooling. 
Nos.  6  and  7  behaved  exactly  like  the  former,  only  requiring  a 
little  more  time,  namely,  5  and  12  minutes  more. 
For  use  in  the  arts,  such  as  the  manufacture  of  printers' 
rollers,  the  commoner  qualities  of  glycerin  are  used,  on  account 
of  their  low  price  and  because  the  small  quantitiy  of  odorous 
principles  and  of  inorganic  constituents  are  without  influence  on 
the  final  result.  These  commoner  qualities  of  glycerin  are 
usually  of  less  specific  gravity  than  the  Pharmacopoeia  requires. 
The  glycerin  used  in  the  experiments  just  related  was,  therefore, 
better  than  the  qualities  likely  to  be  employed,  though,  perhaps, 
of  about  the  same  specific  gravity  as  the  densest  of  the  commoner 
qualities.  Only  two  experiments  were  made  with  a  glycerin  fully 
up  in  every  respect  to  the  requirements  of  the  CJ.  S.  Pharma- 
copoeia. Experiment  No.  8  was  as  follows  :  White  glue  of  the 
same  lot  as  before  was  digested  for  ten  minutes  at  200°  F.,  with 
