The  more  important  tests  in  ordinary  clinical  work  are  those  for  (1)
acidity;  (2)  presence  of  free  acids;  (3)  presence  of  free  HCl,  lactic,  and
butyric  acids;  (4)  presence  of  HCl  in  combination  (combined  HCl).  Tests
for  proteid  digestion,  pepsin  and  peptone  reaction,  are  usually  considered
to  be  unnecessary'  when  normal  or  increased  free  HCl  is  found.  Milk-
curdling  ferment  is  rarely  tested  for.  A  fat-splitting  ferment  in  small
amounts  has  been  occasionally  demonstrated,  but  is  not  regarded  as  of
great  practical  importance.  The  qualitative  tests  in  common  use  can  be
first  considered.
Qualitative  Tests. — 1.  Test  for  Acidity. — The  products  and  agents
of  gastric  digestion  are  normally  acid,  this  reaction  being  due  to  free  acids
— HCl,  lactic  acid,  butyric  acid,  and  their  combinations.  Blue  litmus
paper  is  reddened  by  their  presence.
2.  Tests  for  the  Presence  of  P'ree  Acids. — Congo  red  paper  or
solution  is  turned  deep  blue  by  free  HCl.  A  less  intense  reaction  is  given
by  the  organic  acids.
3.  Tests  for  the  Presence  of  Free  HCl,  Lactic  Acid,  etc. — HCL
— (a)  Methyl  Violet  Reaction, — To  a  pale  violet  solution  of  methyl  violet
(one  drop  concentrated  aciueous  or  alcoholic  solution  in  a  test-tube  of
water)  add  a  few  drops  of  the  filtrate.  A  distinct  blue  change  takes  place
if  free  HCl  is  present.     A  control  tube  should  be  on  hand  for  comparison.
(b)  Tropcvolin  00  Reaction. — Two  or  more  drops  of  fresh  concen-
trated alcoholic  solution  of  tropieolin  (a  deep  orange-colored  solution)
are  spread  on  a  porcelain  plate  or  dish.  The  same  amount  of  filtrate  is
added  to  this  surface  and  the  porcelain  gently  heated.  A  distinct  violet
reaction  turning  to  blue  takes  place.
