Excerpt from Nicotine Entry in 11th Edition of Ency. Britannica

One of the best editions of Encyclopedia Britannica was the
11th edition. It contained information about Adolf Pinner's
method of deducing the structure of Nicotine.
Below is reproduced the first paragraph from p. 666
of v. 19 of the 11th edition of Encyclopedia Britannica.

The constitution of nicotine was established by A. Pinner
(see papers in the Berichte, 1891 to 1895). With bromine in
acetic acid solution at ordinary temperature, nicotine yields
a perbromide, C10H10Br2N2O.HBr3, which with sulphur dioxide,
followed by potash, gives dibromcotinine, C10H10Br2N2O, from which
cotinine, C2H12N2O, is obtained by distillation over zinc dust.
By heating nicotine with bromine in hydrobromic acid solution for
some hours at 100 degrees C, dibromticonine hydrobromide,
C10H8N2Br2O2.HBr, results.  Dibromcotinine on hydrolysis yields
oxalic acid, methylamine, and beta-methyl pyridyl ketone:
C10H10Br2N2O+3H2O+O=H2C2O4+CH3NH2+C5H4N.COCH3+2HBr; whilst
dibromticonine yields methylamine, malonic acid, and nicotinic
acid: C10H8Br2N2O2+4H2O=CH3NH2++(CH2)(CO2H)2+C5H4N.CO2H+2HBr,
or if heated with zinc and caustic potash, methylamine,
and pyridyl-beta gamma-dioxybutyric acid. Thus the groupings

 
 / \-C.C- 
 | |        ,  -C.C- ,  >N-CH3 and -C.C.C-
 \ /
  N
 

exist in the molecule, and the alkaloid is to be represented as
alpha-pyridyl-N-methyl-pyrollidine.

This result has been confirmed by its synthesis by
A. Pictet and P. Crepieux (Comptes rendus, 1903, 137, p. 860)...
... The following formulas are important:

 
/ \-CH.CH2
| | N  CH2
\ /  \//
 N H3C CH2
 
  nicotine