The Fluid State
The most prominent feature of liquids is the ease with which they can be displaced. If one wish to cause a body to move rapidly through water a certain resistance is felt. This resistance is not produced by forces tending to maintain the molecules of the water in their relative positions, but is due to the inertia of the masses of water which have to be thrust aside so quickly. But apart from the inertia of the molecules there are forces which must be overcome when a body is moved through water. It is easy to discern the great differences in mobility which exist between ether, water, oil, glycerine, treacle, etc., a list which could easily be extended. Having passed the very sticky or viscous liquids, substances such as pitch and asphalt occur. On a hot summer's day these are almost fluids, whereas in winter they are hard and brittle.