45  8  Micro-Chemistry  of  Tea  Leaves.  {^'dl^S^' 
thein,  is  poured  on  to  one  of  the  thin  discs  of  glass  already  mentioned, 
and  then  evaporated  nearly  to  dryness  on  the  subliming  plate.  When 
it  approaches  dryness  the  "  subliming  cell  "  is  completed  by  the  circle 
of  glass  and  cover,  and  in  this  way  a  sublimate  is  readily  obtained.  If 
the  substance  is  derived  from  a  thein-producing  plant,  a  distinct  subli- 
mate of  thein  will  be  the  result.  The  leaves,  etc.,  of  the  tea  plant 
also  yield,  without  any  preparation  whatever,  scanty  sublimates  of  thein, 
and  coffee  gives  up  a  very  large  proportion  of  the  alkaloid,  below  no° 
C,  but,  at  all  events,  in  the  case  of  tea  it  is  most  certain  to  operate 
with  magnesia  as  described.  I  may  here  remark  that  if  a  small  quan- 
tity, say  a  gram,  of  finely  powdered  tea  be  placed  between  two  watch 
glasses,  and  heated  in  the  water  bath  in  the  usual  way,  on  removing 
the  upper  glass,  at  the  end  of  an  hour  or  so,  all  round  but  within  the 
edge  crystals  of  thein  can  be  discovered  by  the  microscope.  It  is,  then, 
evident  that  in  the  ordinary  way  of  taking  the  hygroscopic  moisture  of 
tea  there  is  some  loss  of  thein  ;  but  this  is,  I  think,  too  small  to  be 
regarded  in  mere  technical  processes.  I  should  also  add  that  the  addi- 
tion of  magnesia  to  a  decoction  of  tea  or  coffee  for  the  purpose  of 
dialysis  is  not  absolutely  essential,  since  thein  (somewhat  scantily)  dia- 
iyzes  without  the  addition  of  any  reagent.  The  main  objection  to  the 
processes  I  have  given  is  their  extreme  delicacy  ;  any  speck  of  a  tea 
leaf  which  is  easily  visible  to  the  naked  eye  will  yield  its  infinitesimal 
group  of  crystals  to  the  cover  of  the  subliming  cell ;  hence,  in  the 
examination  of  a  foreign  leaf,  any  fragment  of  genuine  tea  mechani- 
cally adhering  to  it  may  give  rise  to  error.  It  must,  however,  be 
borne  in  mind  that  a  great  many  leaves  in  the  vegetable  kingdom  will 
yield,  by  appropriate  treatment,  michrochemical  evidence  as  definite  as 
that  of  tea,  and  the  time  may  come  when  a  large  proportion  of  minute 
vegetable  products  will  be  identified,  not  alone  by  the  shape  of  their 
stomata,  their  epidermal  appendages,  or  the  structure  of  their  ultimate 
vesicles,  but  by  isolating  their  acids,  their  glucosides,  or  their  alkaloids, 
and  evolving  a  microscopical  corpus  delicti  from  a  milligram  of  crude 
material. 
Quantitative  Determination  of  Tbeina. — Struck  with  the  ease  and 
purity  with  which  theina  sublimed,  it  was  but  natural  that  I  should 
attempt  to  work  out  a  quantitative  method  of  sublimation.  I  believe 
I  have  been  successful,  and,  according  to  my  own  repeated  experi- 
ments, the  process  I  give  here  is  both  quick  and  accurate. 
