it　was　accomplished　by　writing　on　paper　saturated
  with　chromates　and　ultramarine。
  In　1871　Professor　Wattenbach　of　Germany　published
  a　treatise　entitled　〃Archives　during　the　Middle
  Ages；〃　which　has　some　valuable　references　to　the　color
  phenomena　of　inks。
  William　Inglis　Clark　in　1879　submitted　to　the　Edinburgh
  University　a　thesis　entitled　〃An　Attempt　to
  Place　the　Manufacture　of　Ink　on　a　Scientific　Basis；〃
  and　which　very　justly　received　the　commendation　of
  the　University　authorities。　His　researches　and　rational
  deductions　are　of　the　greatest　possible　value
  judged　from　a　scientific　standpoint。　The　introduction
  of　blue…black　ink　is　a　phase　of　the　development　towards
  modern　methods　which　he　discusses　at　much
  length。
  The　object　of　adding　a　dye　in　moderation；　he
  asserts；　is　to　give　temporary　color　to　the　ink　and
  where　indigo…paste　is　used；　it　has　been　assumed　that
  it　kept　the　iron　gallo…tannate　in　solution；　whereas　any
  virtue　of　this　kind　which　indigo…paste　possesses　is
  more　likely　due　to　the　sulphuric　acid　which　it　contains
  than　to　the　indigo　itself。　The　essential　part　of　the
  paste　required　is　the　sulpho…indigodate　of　sodium；　now
  commonly　called　indigo…carmine。　He　further　remarks
  that　the　stability　of　an　ink　precipitate　depends　upon
  the　amount　of　iron　which　it　contains　and　which　on　no
  account　should　be　less　than　eight　per　cent；　he　adds
  rightly；　if　gallic　acid　be　preferably　used　in　substitution
  for　tannin；　〃no　precipitate　is　obtained　under
  precisely　similar　conditions。〃　This　point　followed　up
  explains　in　a　measure　why　a　gall　infusion　prepared
  with　hot　water　is　not　suitable　for　a　blue…black；　while
  a　cold　water　infusion　is。　In　the　latter　case　a
  comparatively　small　percentage　of　tannin　is　extracted
  from　the　galls；　while　much　is　extracted　with　hot　water
  and　the　consequence　is；　on　adding　the　indigo　blue　the
  color　is　not　brought　out　as　it　should　be。　Substantially
  the　same　thing　occurs　with　ink　made　with　the　respective
  acids；　although　the　blue　color　remains　for　a　time　unimpaired
  in　the　tannin　ink；　apparently　due　to　the　fact　that
  ferrous…tannate　reduces　indigo　blue　to　indigo　white；　a
  change　which　the　low　reducing　power　of　ferrous…
  gallate　does　little　to　effect。　The　vegetable　matter　in
  common　inks　facilitates　the　destruction；　or　rather
  alteration　and　precipitation　of　the　indigo；　for　the　dye
  appears　in　the　iron　precipitate　and　may　be　extracted
  from　it　with　boiling　water。
  Dr。　Clark's　investigations　seek　to　demonstrate　the
  superiority　of　tannin　and　gallic　acid　over　infusions　of
  the　natural　galls；　and　he　undertakes　to　determine　the
  correct　ratio　of　tannin　and　sulphate　of　iron　to　be　used
  as　ink。　His　experiments　in　this　line　show　that：
  1。　The　amount　of　precipitate　increases　as　the　proportion
  of　iron　to　tannin　is　increased。
  2。　The　composition　of　the　precipitate　is　so　valuable
  as　to　preclude　the　possibility　of　its　being　a　definite
  body。　Increase　of　iron　in　the　solution　has　not　at　first
  any　effect　on　the　composition　of　the　precipitate；　but
  afterwards　iron　is　found　in　it　in　greater　but　not　proportional
  amount。
  3。　At　one　point　the　proportions　of　iron　in　the　precipitate
  and　in　solution　are　the　same；　and　this　is　at
  between　6　and　10　parts　of　iron　to　100　parts　of　tannin。
  4。　The　proportion　of　iron　in　the　precipitate　varies
  greatly　with　the　length　of　time　the　ink　has　been　exposed。
  At　first　the　precipitate　contains　10　per　cent
  of　iron；　but　by　and　by　a　new　one　having　only　7。5
  per　cent　is　formed；　and　in　from　forty　to　seventy　days
  we　find　one　of　5。7　per　cent。　Simultaneously　iron　increases
  in　the　ink　（proportionate　to　the　tannin）。
  5。　The　results　show；　and　practice　confirms；　that
  16　parts　of　iron　（80　ferrous　sulphate）　and　100　parts　of
  tannin　are　best　for　ink　manufacture。
  The　research　now　travelled　in　a　direction　which
  accumulating　experience　showed　to　be　obligatory。
  Blue…black　tannin　ink　lost　color；　and　the　reducing
  nature　of　the　tannin　tended　to　the　formation　of　a
  highly　objectionable　precipitate　in　the　ink；　which
  made　writing　anything　but　a　pleasure。　These　two
  faults　were　doubtless　linked　together　in　some　way
  and　seemed　not　to　exist　when　gallic　acid　was　used；
  for　ink　so　made　was　found　to　precipitate　only　after
  a　long　exposure；　it　required　no　free　acid　to　keep　the
  precipitate　in　solution；　and　retained　the　indigo　blue
  color　for　a　long　time；　alkalis　did　not　decompose　the
  ink；　and　provided　blacker　and　more　permanent　writing。
  Determination　of　the　correct　proportions　of
  gallic　acid　and　ferrous…sulphate　was　the　subject　of　prolonged
  experiments　conducted　on　similar　lines　to　those
  already　detailed。　The　conclusions　as　to　precipitation
  were　also　similar。　Thirty　parts　of　iron　（150　of　ferrous…
  sulphate）　and　100　parts　of　gallic　acid　were　found　to
  be　the　most　suitable　proportions　for　ink…making。　It
  is　advisable；　however；　not　to　discard　tannin　altogether；
  owing　to　the　slow　blackening　of　the　gallic　acid　ink；
  and　a　little　tannin　gives　initial　blackening　and　body；
  while　it　is　absolutely　necessary　for　copying　ink。
  Initial　blackness　can　also　be　ensured　by　oxidizing
  21　per　cent　of　the　ferrous…sulphate　without　adding
  the　extra　acid　necessary　to　the　formation　of　a　ferric
  salt。
  The　concluding　portion　of　his　research　is　devoted
  to　the　influence　of　sugar　upon　the　permanence　of　ink；
  and　the　results　of　the　experiments　are　summed　up　in
  the　following　sentences：　〃It　would　be　injurious　to
  add　3　per　cent　of　sugar　to　a　tan　in　ink；　while　from
  4　to　10　per　cent　would　be　quite　allowable。　Most
  copying　inks　contain　about　3。5　per　cent　of　sugar
  not　far　from　the　critical　amount。　With　gallic　acid
  more　than　3　per　cent　of　sugar　hardly　varies　the　precipitate；
  but　the　importance　of　this　point　is　somewhat
  diminished　by　the　fact　that　the　presence　of　sugar　is
  by　no　means　necessary　in　a　writing　ink。　Dextrin　is
  a　much　superior　substance　to　use。　Curiously　this
  body　rapidly　precipitates　a　tannin　ink；　hence　it　is
  useless　for　copying　ink；　but　for　the　gallic　ink　it　is　an
  excellent　thickener。〃
  Chen…Ki…Souen；　〃Lencre　de　China；〃　by　Maurice　Jametel；
  appeared　in　Paris　in　1882；　but　as　the　title　indicates；
  it　is　the　old　〃Indian〃　or　Chinese　ink　that　is　discussed。
  Schluttig　and　Neumann　in　1890　issued　their
  Edition　Dresden　on　the　subject　of　〃Iron　and　Gall
  inks。〃　In　this　valuable　work　is　to　be　found　the
  formula　which　has　been　generally　adopted　as　the
  standard　where　one　is　used　for　tanno…gallate　of　iron　ink。
  The　investigations　of　other　scientific　men　like　Lepowitz；
  Booth；　Desormeaux；　Chevreuse；　Irvine；　Traille；
  Bottger；　Riffault；　Precht；　Nicholes；　Runge；　Gobert；
  Penny；　Arnold；　Thomson　（Lord　Kelvin）；　Davids；　Kindt；
  Ure；　Wislar　and　many　more　who　have　dealt　with　the
  chemistry　of　inks；　present　to　us　some　testimony　during
  a　considerable　portion　of　the　nineteenth　century
  of　the　efforts　made　to　secure　a　good　ink。
  CHAPTER　XIV。
  CLASSIFICATIONS　OF　INK。
  INK　USED　BY　US　HAS　NOTHING　IN　COMMON　WITH　THAT
  OF　THE　ANCIENTSMANUFACTURERS　OF　THE　PRESENT
  TIME　HAVE　LARGELY　UTILIZED　FORMULAS　EMPLOYED
  IN　PAST　CENTURIESTHE　COMMON　ACCEPTATION　OF
  THE　TERM　INKSEVEN　DIFFERENT　CLASSES　OF　INKS
  AND　THEIR　COMPOSITION　BRIEFLY　TOLDFAILURE　OF
  EFFORTS　TO　SECURE　A　REAL　SAFETY　INK。
  THE　inks　used　by　us　have　nothing　in　common　with
  those　of　the　ancients　except　the　color　and　gum；　and
  mighty　little　of　that。
  Those　of　the　〃gall〃　class　employed　in　the　fourteenth；
  fifteenth；　sixteenth；　seventeenth　and　eighteenth
  centuries；　some　formulas　of　which　are　utilized
  by　the　manufacturers　of　ink　in　our　own　time；　consisted
  generally　in　combination；　infusions　of　nut…galls；　sulphate
  of　copper　or　iron；　or　both；　and　fish…glue　or　gum；
  slightly　acidulated。　The　frequent　introduction　of　the
  so…called　〃added〃　color　into　these　inks；　time　has　shown
  to　have　been　a　grave　mistake。
  The　common　acceptation　of　the　term　〃ink〃　may　be
  said　to　characterize　an　immense　number　of　fluid　compounds；
  the　function　of　which　in　connection　with　a
  marking　instrument　is　to　delineate　conventional　signs；
  characters　and　letters　as　put　together　and　commonly
  called　writing；　on　paper　or　like　substances。
  To　classify　them　would　be　impossible；　but　black
  writing　ink；　chemical　writing　fluid；　colored　writing
  ink；　copying　ink；　India　ink；　secret　or　sympathetic　ink；
  and　indelible　ink　make　seven　classes；　the　others　may
  be　denominated　under　the　head　of　miscellaneous　inks；
  and　of　them　all；　there　is　no　single　ink　answering　every
  requirement　and　few　answer　at　all　times　the　same　requirements。
  Ink　may　be　either　a　clear　solution　of　any
  coloring　matter　or　of　coloring　matter　held　in　suspension。
  It　is　a　remarkable　fact　that　although　most　inks
  are　chemical　compositions　and　many　times　made　after
  the　same　formula；　identical　results　cannot　always　be
  calculated　or　obtained。　This　is　more　particularly　to　be
  noted　in　the　case　of　black　writing　inks　otherwise
  know