52 
ON  EFFERVESCING  POWDERS. 
If  the  pure  sulphate  of  the  protoxide  is  mixed  with  tartaric 
acid,  the  mixture  keeps  admirably  well,  so  far  at  least  as  may 
be  judged  from  the  color,  and  in  making  such  an  effervescing 
draught,  the  iron  will  consequently  reach  the  stomach  before  it 
can  be  oxidized  to  sesquioxide  by  the  influence  of  the  atmos- 
phere. 
In  the  preceding  I  have  taken  a  cursory  view  of  the  various 
kinds  of  those  effervescing  powders,  which  are  put  up  with  the 
acid  and  the  carbonate  in  different  parcels,  and  will  now  pro- 
ceed to  the  principal  object  of  this  paper,  that  of  drawing 
attention  to  that  very  convenient  form  of  the  same  kind  of 
powders,  which  differ  from  the  former  in  containing  all  the  ma- 
terials necessary  for  the  evolution  of  carbonic  acid  gas  mixed  into 
one  uniform  powder.  Of  these  effervescing  powders,  Wood  and 
Bache's  Dispensatory  has  on  page  54  the  following  short  notice  : 
«  Tartaric  acid,  dried  by  a  gentle  heat  and  then  mixed  in  due 
proportion  with  bicarbonate  of  soda,  forms  a  good  effervescing 
powder,  a  teaspoonful  of  which,  stirred  into  a  tumbler  of  water 
forms  the  dose.  The  mixture  must  be  kept  in  well  stopped  vials." 
Every  body  who  has  been  making  such  powders,  will  have 
experienced  that  such  a  mixture,  no  matter  how  closely  stopped 
it  may  be  kept?  will  spoil  and  even  become  moist.  Professor 
Otto  has  made  some  experiments  and  discovered  some  interest- 
ing facts  with  regard  to  this  phenomenon,  an  account  of  which 
he  has  published  in  the  Annalen  der  Chemie  und  Pharmacie, 
xvii,  378,  of  which  I  will  give  a  short  abstract.  A  mixture 
of  equal  parts  of  bicarbonate  of  soda  and  tartaric  acid  becomes 
moist  when  introduced  into  well  stopped  vials,  and  this  change 
takes  place  the  quicker,  the  better  the  air  is  excluded ;  it  keeps 
better  if  the  vials  are  simply  covered  with  paper ;  but  if  the 
powder  is  kept  in  ordinary  paper  boxes,  he  has  never  seen  it  to 
spoil.  Experiments  of  Bosse  have  shown  that  such  a  mixture 
loses  weight  from  the  expulsion  of  carbonic  acid,  which  loss  is 
greater,  the  better  the  powder  is  secured  from  contact  with  the 
air.  This  decomposition  is  introduced  by  a  portion  of  moisture 
which  the  mixture  contains,  and  which  must  evaporate  on  the 
air,  to  keep  the  powder  unaltered.  But  if  both  ingredients  are 
dried  before  mixing  at  a  temperature  between  120  and  145°  F. 
