It means that the same mass of a body, under the same circumstances,
always requires the same quantity of heat to raise it to a given
temperature, but that equal masses of different bodies require
different quantities. Ultimately, it was found that the quantities of
heat required to raise equal masses of the more perfect gases, through
equal ranges of temperature, were inversely proportional to their
combining weights. Thus a definite relation was established between
the hypothetical units and heat. The phenomena of electrolytic
decomposition showed that there was a like close relation between
these units and electricity. The quantity of electricity generated by
the combination of any two units is sufficient to separate any other
two which are susceptible of such decomposition. The phenomena of
isomorphism showed a relation between the units and crystalline forms;
certain units are thus able to replace others in a crystalline body
without altering its form, and others are not.

Again, the laws of the effect of pressure and heat on gaseous bodies,
the fact that they combine in definite proportions by volume, and that
such proportion bears a simple relation to their combining weights,
all harmonised with the Daltonian hypothesis, and led to the bold
speculation known as the law of Avogadro--that all gaseous bodies,
under the same physical conditions, contain the same number of units.
In the form in which it was first enunciated, this hypothesis was
incorrect--perhaps it is not exactly true in any form; but it is
hardly too much to say that chemistry and molecular physics would
never have advanced to their present condition unless it had been
assumed to be true. Another immense service rendered by Dalton, as a
corollary of the new atomic doctrine, was the creation of a system of
symbolic notation, which not only made the nature of chemical
compounds and processes easily intelligible and easy of recollection,