388 
ON  SOME  POINTS  OF  CHEMICAL  THEOKY,  ETC. 
make  88,  and  this  number,  divided  by  2,  gives  44,  the  equivalent 
of  strontium.  Once  more,  7  and  39,  the  equivalents  of  lithium 
and  potassium,  added  together,  make  46,  which  number,  when 
halved,  gives  23,  the  equivalent  of  sodium.  Chlorine,  bromine 
and  iodine  are  three  elements,  connected  by  the  closest  affinities. 
Here,  according  to  Dumas,  we  have  the  equivalents,  35»5  for 
chlorine,  80  for  bromine,  and  127  for  iodine.  Let  us  see 
whether  these  numbers  support  the  law.  They  certainly  do  not, 
very  exactly  ;  for,  to  confirm  the  law,  the  equivalent  of  bromine 
should  be  81-25,  instead  of  80.  Nevertheless,  the  departure 
from  conformity  to  the  law  is  not  very  great  ;  and,  when  we 
reflect  how  nice  a  problem  it  is  to  obtain  the  equivalents  of 
bromine  and  iodine  exactly,  from  the  difficulty  of  purifying  these 
elements,  we  may  be  permitted  to  suppose,  that  new  determina- 
tions may  possibly  alter  the  equivalents,  80  and  127,  for 
bromine  and  iodine ;  although  Dumas  states  that  he  obtained 
these  numbers  after  the  most  careful  experiments. 
Before  leaving  this  subject,  I  will  mention  a  kindred  one; 
namely,  that  two  equivalents  have,  sometimes,  the  simple  rela- 
tion to  one  another  of  one  to  one,  or  of  one  to  two.  Thus,  man- 
ganese and  chromium  have,  according  to  Dumas,  the  same 
equivalent  number  (26),  and  we  all  know  that  the  equivalent  of 
sulphur  is  exactly  double  that  of  oxygen. 
Perhaps  I  have  occupied  too  much  space  with  these  curious 
speculations  respecting  the  equivalent  numbers.  Hypotheses, 
however,  are  not  without  their  use,  and,  if  they  lead  us  to 
interrogate  nature  by  experiment,  are  often  productive  of 
fruit.  Theories  are  more  useful  than  hypotheses,  because  they 
embrace  an  extended  generalization  of  facts,  or  supposed  facts ; 
still,  they  are  liable  to  be  changed  with  the  progress  of  our 
knowledge.  "A  theory  in  chemistry,"  as  Gregory  has  very 
happily  expressed  it,  "is  nothing  more  than  such  a  view  as 
groups  together  the  largest  number  of  facts  at  a  given  time ; 
ani  has  no  pretensions  to  absolute  truth,  nor  to  be  more  than  a 
guide  to  the  inquirer,  and  an  aid  to  his  memory.,, 
One  of  the  most  important  sets  of  compounds,  known  to  the 
chemist,  is  the  series  of  homologous  carbohydrogens,  each  con- 
taining a  progressively  increasing  amount  of  carbon,  represented, 
in  equivalents,  by  the  even  numbers,  2,  4,  6,  8,  &c,  up  to  60; 
