These reach back to the most remote antiquity, and to follow their development we should have to write the history of human thought which they have always accompanied since the time of Leucippus, Democritus, Epicurus, and Lucretius.

The first observers who noticed that the volume of a body could be diminished by compression or cold, or augmented by heat, and who saw a soluble solid body mix completely with the water which dissolved it, must have been compelled to suppose that matter was not dispersed continuously throughout the space it seemed to occupy.

They were thus brought to consider it discontinuous, and to admit that a substance having the same composition and the same properties in all its parts--in a word, perfectly homogeneous--ceases to present this homogeneity when considered within a sufficiently small volume.
Modern experimenters have succeeded by direct experiments in placing in evidence this heterogeneous character of matter when taken in small mass.

Thus, for example, the superficial tension, which is constant for the same liquid at a given temperature, no longer has the same value when the thickness of the layer of liquid becomes extremely small.

Newton noticed even in his time that a dark zone is seen to form on a soap bubble at the moment when it becomes so thin that it must burst.