ON  SOME  POINTS  OF  CHEMICAL  THEORY,  ETC. 
389 
with  but  few  chasms  in  the  series,  and  these  are  filling  up  gradu- 
ally by  new  discoveries.  In  these  compounds  the  hydrogen 
uniformly  exceeds  the  carbon,  in  number  of  equivalents,  by  one 
equivalent ;  and,  consequently,  is  represented  by  equivalents, 
which  gradually  rise  by  the  odd  numbers,  3,  5,  7,  9,  &c,  up  to 
61.  These  carbohydrogens  are  positive  radicals,  each  of  which 
has  a  protoxide,  that  is,  an  ether  ;  and  also  a  hydrated  protox- 
ide, that  is,  an  alcohol.  Take  from  each  of  these  positive  radi- 
cals, two  equivalents  of  hydrogen,  which  makes  the  hydrogen 
fall  below  the  carbon,  in  number  of  equivalents,  by  one  equiva- 
lent, and  they  are  converted  into  negative  radicals,  each  of  which 
has  a  derived  acid,  all  of  similar  constitution,  being  uniformly 
hydrated  teroxides.  Of  the  positive  radicals,  the  first  five  in 
the  series,  namely,  methyl,  ethyl,  propyl,  butyl  and  amyl,  are 
the  most  deserving  of  study ;  because  they  form  a  key  to  the 
knowledge  of  the  principal  ethers  and  alcohols. 
In  order  to  introduce  to  your  notice  the  important  doctrine  of 
chemical  substitution,  which  has  been  so  fertile  of  new  discov- 
eries, I  must  remind  you  of  the  composition  of  the  compounds  of 
nitrogen  and  hydrogen.  Ammonium  is  a  compound  radical, 
which  consists  of  one  equivalent  of  nitrogen  and  four  of  hydro- 
gen. Withdraw  the  hydrogen  gradually,  one  equivalent  at  a 
time,  and  you  have,  first  ammonia,  then  amide,  and,  finally, 
imide  ;  the  latter  being  a  compound  of  single  equivalents  of  nitro- 
gen and  hydrogen.  It  is  only  lately  that  imide  has  been  recog- 
nised under  a  distinct  name.  It  is  now  many  years  since  I  point- 
ed out  in  my  lectures,  that  such  a  compound  exists  in  hydrocyanic 
acid,  associated  with  carbon,  and  mentioned  how  important  it 
would  be,  for  the  elucidation  of  the  laws  of  combination,  to  ob- 
tain it  in  a  separate  state  ;  since  it  is  composed  of  single  equiva- 
lents of  nitrogen  and  hydrogen.  This  compound,  though  now 
named,  has  not,  indeed,  been  isolated,  but  may  be  transferred 
to  different  substances,  so  as  to  form  various  combinations. 
Now,  ammonia  is  a  fertile  source  of  substitution-compounds. 
It  may  have  one,  two,  or  all  of  its  equivalents  of  hydrogen 
replaced  by  certain  compound  radicals,  which  fit,  as  it  were,  the 
place  previously  occupied  by  the  hydrogen.  We  have,  when  one 
equivalent  of  hydrogen  is  replaced,  amide  bases ;  when  two  equi~ 
valents  are  replaced,  imide  bases;  when  three  are  replaced,  that 
