^^'jan^,\sm^^^'}    Action  of  Acids  upon  Zinc  Containing  Lead.  23 
which  are  the  initial  velocities  whilst  the  abscissae  are  the  temperatures. 
These  curves  seem  to  be  asymptotic  to  the  axis  of  the  temperatures^ 
aud  the  three  curves  for  the  three  different  degrees  of  concentration  of 
the  acid  converge  at  a  point  which  corresponds  w^ith  a  temperature  of 
-60°  to  -70°.  This  result  seems  to  indicate  that  at  a  temperature 
below  -70°  hydrochloric  acid  will  have  no  action  on  zinc  whatever  the 
concentration  of  the  acid,  and  it  is  of  interest  to  recall  the  well-known 
fact  that  liquid  hydrogen  chloride,  which  liquefies  at  about  -70°,  has 
no  action  on  this  metal. 
According  to  Kohlrausch,  the  maximum  conductivity  of  hydro- 
chloric acid  corresponds  with  a  strength  of  21  per  cent.  Acid  of  25, 
30,  or  34  per  cent.,  however,  dissolves  zinc  more  quickly  than  acid  of 
21  per  cent.,  and  from  this  and  the  previous  results  it  is  evident  that 
the  conclusion  of  De  la  Rive  and  of  Kajander,  that  the  rate  of  solu- 
tion is  intimately  connected  with  the  electrical  conductivity  of  the 
acid,  is  not  confirmed. 
With  hydrobromic  acid,  the  rate  of  solution  is  much  higher  than 
with  hydrochloric  acid  of  corresponding  concentration.  Under  ordi- 
nary conditions  the  curve  is  not  a  right  line  at  any  phase  of  the  reac- 
tion, a  result  due  to  the  fact  that  with  an  increased  rate  of  solution 
the  difference  between  the  temperature  of  the  liquid  and  that  of  the 
metal  is  much  greater,  and  the  perturbations  due  to  this  cause  are 
greatly  increassd.  If  the  conditions  are  such  that  the  temperature  of 
the  zinc  is  kept  constant,  it  is  found  that  the  period  of  induction  is 
very  short,  the  velocity  of  solution  rapidly  attains  a  maximum,  and 
then  decreases  with  the  concentration  of  the  solution,  the  latter  part 
of  the  curve  being  a  right  line.  It  is  a  general  result  that  the  period 
of  induction  is  shorter  the  greater  the  concentration  of  the  acid.  With 
hydrochloric  acid  of  30  per  cent.,  for  example,  there  is  practically  no 
period  of  induction,  and  the  maximum  velocity  is  attained  at  once. 
The  tlieoretical  initial  velocity  with  hydrobromic  acid  is  2.29  times 
that  with  hydrochloric  acid. 
With  hydriodic  acid,  the  velocity  during  the  period  of  induction  is 
less  than  in  the  case  of  hydrochloric  acid,  and  the  difference  is  greater 
the  weaker  the  acids,  but  after  the  maximum  velocity  is  attained  the 
rate  of  solution  is  the  same  for  both  acids,  and  if  the  curves  are 
drawn  on  a  small  scale  they  coincide.  It  follows,  of  course,  that 
the  theoretical  initial  velocities  are  practically  the  same  for  both 
acids. 
