based on no trivial or accidental features of the facts, but on what
has proved to be the very essence of the phenomena they sought to
bring into one point of view; for all the advances made in our own
times in clear knowledge of the transformations of matter have been
made by using, as a guide to experimental inquiries, the conception
that the differences between the qualities of substances are connected
with differences in the weights and movements of minute particles; and
this was the central idea of the atomic theory of the Greek
philosophers.

The atomic theory was used by the great physicists of the later
Renaissance, by Galileo, Gassendi, Newton and others. Our own
countryman, John Dalton, while trying (in the early years of the 19th
century) to form a mental presentation of the atmosphere in terms of
the theory of atoms, rediscovered the possibility of differences
between the sizes of atoms, applied this idea to the facts concerning
the quantitative compositions of compounds which had been established
by others, developed a method for determining the relative weights of
atoms of different kinds, and started chemistry on the course which it
has followed so successfully.

Instead of blaming the Greek philosophers for lack of quantitatively
accurate experimental inquiry, we should rather be full of admiring
wonder at the extraordinary acuteness of their mental vision, and the
soundness of their scientific spirit.

The ancient atomists distinguished the essential properties of things
from their accidental features. The former cannot be removed,
Lucretius said, without "utter destruction accompanying the
severance"; the latter may be altered "while the nature of the thing