Am.  Jour.  Pharm.  ) 
November,  1920.  ) 
Book  Reviews. 
843 
this  phase  of  the  subject,  a  large  part  of  the  earUer  data  of  soil 
analysis  may  not  have  to  be  scrapped.  Another  point  about  hydro- 
gen ion  concentration  is  that  at  present  the  determination  of  it  is 
among  the  most  satisfactory  of  ordinary  laboratory  processes.  Of 
course,  as  Clark  points  out,  there  is  danger  of  making  the  subject 
a  fetish,  by  which  undue  weight  may  be  given  to  a  secondary  phase 
of  a  problem,  or  a  generalization  much  wider  than  is  justified  be 
made.  There  is  always  a  tendency  to  make  too  much  of  the  new 
view. 
A  commendable  feature  of  the  book,  although  it  may  seem,  at 
first,  merely  minor  detail,  is  the  symbol  used  for  the  factor  of  con- 
centration. Sorensen,  to  whom  is  due  so  much  of  the  development 
of  the  matter,  introduced  a  symbol  somewhat  complicated  in  typo- 
graphic form,  and  hence  some  difference  is  noted  in  the  manner  in 
which  it  is  written  and  printed  by  different  authorities.  Leeds  and 
Northrup,  in  a  recent  publication,  use  the  original  Sorensen  form, 
but  Clark  uses  the  simpler  form,  Ph.  This,  also  used  by  the  Journal 
of  Biological  Chemistry,  is  much  more  easy  for  the  compositor  and 
fully  as  satisfactory  to  the  chemist.  There  seems  to  be  no  reason 
for  retaining  a  complex  expression  merely  because  it  is  the  form 
originally  suggested. 
The  procedures  given  in  the  book  are  wisely  limited  to  the  two 
types  by  which  the  concentration  determinations  are  made  in  the 
working  laboratory,  that  by  indicators  and  that  by  electrometric 
methods.  Both  these  methods  are  described  in  great  detail.  The 
theory  of  indicators  has  been  the  subject  of  extensive  study  among 
chemists,  and  the  phenomena  of  color  change  under  the  differences 
of  acidity  and  alkalinity  are  striking  even  to  the  experienced  worker. 
The  chemical  and  physical  changes  occurring  in  many  indicators 
are  complicated,  involving  even  the  reactions  of  tautomerism.  Ad- 
vance in  the  practical  application  of  these  substances  has,  however, 
brought  out  the  fact  that  by  the  use  of  what  are  termed  "buffer" 
solutions,  indicators  may  be  classed  empirically,  i.  e.,  without 
taking  into  consideration  any  theory.  These  buffer  solutions  are 
standard  solutions  of  such  well  defined  composition  and  hydrogen 
ion  concentration  that  they  can  be  accurately  duplicated.  For  the 
highest  accuracy,  the  degree  of  ion  concentration  should  be  verified 
by  the  electrometric  method,  which  is  an  absolute  one.  They  gen- 
erally consist  of  a  mixture  of  some  acid  and  one  of  its  alkali  salts, 
Clark  and  Lubs  have  designed  a  set  of  these,  the  details  of  which 
