362 



THE GEOLOGIST. 



to a limpid state, and vice versa, under varying circumstances of 

 temperature oud pressure. But there is au intermediate condition of 

 imperfect liquidity, in which the component particles of a substance 

 possess only a modified mobility, 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 Vv^hat are called viscous substances. Where 

 the substance is not homogeneous, but consists of more or less solid 

 particles mixed or suspended in a liquid, the mobility of the 

 particles and the liquidity of the m.ass, 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 j 

 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 j) 

 rate and in the same direction. Either from the movement being i| 

 communicated to one part before it reaches another, or owing to the 

 resistance offered by the containing surfaces to the motion of the con- l-i 

 tiguous particles, and propagated through them to the others with which [1 

 they are in contact, and so on, irregularities of pressure, and, therefore, |^ 

 of mutual friction, are produced, and, consequently, unequal rates of k 

 motion in different threads or veins of the liquid. Hence all the (i 

 phenomena of currents and eddies. I 



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 jr 

 motion of the more mobile particles in contact with, or contiguous to ti 

 them, and occasion similar internal irregularities of motion. They move jf 

 together with, but less rcadih*, and eousequently more slowly than, the |^ 

 latter, which, by the pressure and friction thus occasioned, are squeezed, j 

 as it were, from among the coarser or more solid particles, and jf 

 take a line of their own, moving more rapidly than the former, while |" 



