John Dalton concluded that gases were made up of solid particles with empty space between them.

This was a key part of his atomic theory, which stated:

* All matter is composed of atoms, which are indivisible and indestructible particles.

* Atoms of a given element are identical in size, mass, and other properties.

* Atoms of different elements differ in size, mass, and other properties.

* Chemical reactions involve the combination, separation, or rearrangement of atoms.

Dalton's idea of empty space between particles explained why gases were compressible and why they diffused more easily than liquids or solids.