With this idea firmly rooted in his mind, modern man had no difficulty in using it to explain both thermal expansion and the effect of heat on the different states of matter, and so, finally, these states themselves. Thermal expansion was thus attributed to an increase in the average distance between the assumed minute particles, caused by an increase in their rate of movement; the liquid state was held to differ from the solid, and similarly the gaseous from the liquid, by the interspaces between the particles becoming relatively so great that the gravitational pull between them became too weak to hold them together.