It  follows  that  while  such  experiments  as  those  conducted  by  Mr.  Sharpies  are  necessary  to  establish
maximum  and  relative  values  for  any  species,  these  being  established,  actual  values  of  any  given  specimen  of


THE  WOODS  OF  THE  UNITED  STATES.
265


wood  may  be  determined  by  microscopic  examination  of  its  structure ;  that  is,  two  specimens  of  the  wood  of  any
species  to  which  the  census  tests  have  been  applied  being  given,  their  relative  values  can  be  determined  by  an
examination  of  their  structure  as  well  as  or  better  than  by  any  elaborate  experiments.
TANSm  VALUES.
The  amount  of  tannin  contained  in  the  bark  of  various  trees  of  the  United  States  has  been  determined.
These  determinations  give  the  proportion  of  tannin.  They  do  not  indicate  the  real  value  of  the  bark  of  the
species  for  tanning,  which  can  only  be  obtained  by  actual  experiments  made  on  a  large  scale,  other  properties  in
the  bark,  beside  the  percentage  of  tannin,  affecting  the  value  of  the  leather  prepared  with  it.
These  determinations  must  therefore  be  regarded  as  approximations,  which  will  serve,  in  some  cases,  to
indicate  species  not  now  in  general  use  for  this  purpose,  which  may  be  looked  to  as  possible  sources  of  tannin
supply.
