Page:The fairy tales of science.djvu/92

. Let us now consider the aggregation of these little bits into masses.

The force which holds the atoms together is called cohesion; it is greater in solids than in liquids, while in aëriform bodies it seems to be altogether absent.

We have every reason to believe that the ultimate particles of a body are never in actual contact, but are placed at a certain distance from each other, so that there exists around every individual particle a space void of matter. All bodies are more or less compressible, and unless we acknowledge the existence of these empty spaces we must suppose that two or more particles are capable of occupying the same place at the same time: a supposition which is opposed to the notion of an atom having a definite size.

A volume of air can be compressed into a space a thousand times smaller than that which it originally occupied, and we must therefore conclude that the atoms of air are separated by wide intervals. Solids and liquids must also have interstices or pores between their particles, as they invariably expand when heated and contract when exposed to a low temperature.

The porosity of gold was demonstrated some two hundred years ago by the famous Florentine experiment. A hollow ball of the precious metal, filled with water, was submitted to a great pressure, by which the fluid was made to ooze through its pores and bedew its outer surface.