362 
THK GEOLOGIST. 
to a liquid state, and vice vorsA, under varying circumstances of 
temperature and pressure. Eut there is an intermediate condition of 
imperfect liquiditj', in which the component particles of a substance 
possess only a modified mobilit)", or capability of movement inter se 
under sufficient pressures. Some substances, apparently homogeneous, 
exhibit this imperfect mobility at ordinary temperatures ; for example, 
honey, pitch, and generally what are called viscous substances. "Where 
the substance is not liomogeneous, but consists of more or less solid 
particles mixed or suspended in a liquid, the mobility of the 
particles and the liquidity of the mass, of course, very much depend 
on the proportion of the solid to the liquid parts, as well as on the size 
and shape of the former, and the degree of fluidity of the latter 
elements. 
Even in the most liquid substances the mobility of the particles is 
attended by more or less of friction, occasioned by their mutual pressure. 
When the particles are homogeneous, equal in size, and spherical, as is 
probably the case in all perfect fluids, the friction accompanying their 
motion, other circumstances remaining the same, will be least. Where 
all, or any considerable proportion of them, are unequal in size, or 
irregular in form, the friction must be proportionately great. 
"When ordinary liquids, such as water, are put in motion by any 
force, the whole body of liquid does not move together at the same 
rate and in the same direction. Either from the movement being 
communicated to one part before it reaches another, or owing to the 
resistance offered by the containing surfaces to the motion of the con- 
tiguous particles, and propagated through them to the others with which 
they are in contact, and so on, irregularities of pressure, and, therefore, 
of mutual friction, are produced, and, consequently, unequal rates of 
motion in different threads or veins of the liquid. Hence all the 
phenomena of currents and eddies. 
When coarse or solid particles of matter are suspended or mixed in a 
moving liquid, they likewise offer more or less of resistance to the 
motion of the more mobile particles in contact with, or contiguous to 
them, and occasion similar internal irregularities of motion. They move 
together with, but less readily, and consequently more slowly than, the 
latter, v/hich, by the pressure and friction thus occasioned, are squeezed, 
a,s it were, from among the coarser or more solid particles, and 
take a line of their own, moving more rapidly than the former, while 
