In an article published recently in this journal, one of us reconstruc
ted how in 1899 J. J. Thomson, after having measured the mass-to-charg
e ratio of the corpuscle (which became our electron), achieved a measu
rement of its charge and consequently an estimate of its mass, obtaini
ng in this manner 'direct proof of the existence of particles smaller
than the hydrogen atom'. In this paper, starting with an analysis of Z
eeman's first measurements on the widening of spectral lines in a magn
etic field, we show that the existence of masses smaller than the atom
was already a matter of discussion in early 1897, and that a qualitat
ive estimate of the mass of the electron was made even before Thomson'
s measurements in 1899. In this process an important role was played b
y a research programme on the structure of matter carried out by Stone
y in the latter part of the nineteenth century. This had in many aspec
ts anticipated Zeeman's experimental observations, and its conclusions
were in part formalized in the so-called Larmor theorem.