Am.  Jour.  Pharm.  \ 
Jan.,  1884.  j 
Oxidisable  Constituents  of  Plants. 
49 
instrument  of  bubbles  of  air  which  cling  persistently  to  the  sides  of 
the  bulb  and  stem. 
The  changes  of  barometric  pressure  may  in  ordinary  cases  be 
neglected,  but  it  is  necessary  to  take  account  of  the  differences  of 
pressure  dependent  upon  the  horizontal  or  vertical  position  of  the 
column  of  mercury  in  the  stem  of  a  long  thermometer.  But  this 
factor  cannot  safely  be  neglected  for  second  determination  under 
reduced  pressures,  when  the  thermometer  is  immersed  in  the  vapour  : 
in  these  cases  it  is  preferable  to  introduce  a  thermometer  in  a  tube 
sealed  at  its  lower  extremity,  and  communicating  directly  with  the 
atmosphere. 
In  conclusion,  the  author  points  out  the  errors  in  determination  of 
boiling  and  melting  points.  In  the  former,  errors  frequently  arise 
from  a  too  hasty  observation;  to  ensure  accuracy  the  whole  stem  must 
be  immersed  in  the  vapour,  and  distillation  must  be  carried  on  for  at 
least  ten  minutes  before  the  whole  of  the  stem  acquires  the  temperature 
of  its  environment. 
To  determine  fusing  points  it  is  preferable  to  plunge  the  thermo- 
meter into  the  melting  substances  and  to  observe  the  changes  of  tem- 
perature during  complete  solidification  :  when  the  quantity  of  sub- 
stance does  not  admit  of  this  method  of  procedure,  the  usual  process 
must.be  adopted. 
In  the  memoirs,  tables  of  corrections  are  given  for  converting  read- 
ings of  ordinary  thermometers  and  those  with  limited  scale,  into  read- 
ings of  the  hydrogen  thermometer. — Jour.  Chem.  Soc,  Sept.,  1883 ; 
Bull.  Soc.  Chim.  [2]  39,  p.  196. 
EASILY  OXIDISABLE  CONSTITUENTS  OF  PLANTS. 
By  J.  Reinke. 
It  is  a  well-known  fact  that  the  juices  of  many  plants  become  discolored 
on  exposure  to  the  air.  So,  too,  sections  of  stems  and  roots  of  leaves  and 
fleshy  fruits  which  acquire  a  brown  color  on  exposure.  Little  has  been 
ascertained  in  regard  to  the  physiology  of  these  changes.  They  obviously 
depend  upon  the  oxidation  of  certain  constituents  ;  this  is  seen,  for  instance, 
on  exposing  grated  potatoes  to  the  air,  when  the  uppermost  layer  assumes  a 
brown  color,  which  by  frequent  turning  over  of  the  mass  may  be  com- 
municated throughout.  The  same  is  seen  in  the  case  of  the  expressed  juice 
of  the  potato.  Putrefaction  or  fermentation,  and  reducing  agents,  such  as 
sulphurous  or  hydrosulphuric  acid,  decolorise  these  fluids.  The  juice  of 
the  white  sugar-beet  is  even  more  sensitive,  becoming  on  exposure  to  the 
4 
