W. H. Brewer—Suspension and Sedimentation of Clays. 3 
of these liquid strata in the aggregate than are visible at any 
one time. 
These different strata, having unlike degrees of opacity, 
settle with very different degrees of rapidity, and if the temper- 
ature be kept nearly constant and the vessel entirely at rest, 
their number grows less and less from the more rapid settlin 
of the lower and heavier. The lighter ones usually settle with 
extreme slowness often but a millimeter per day, some even less 
than half that, and consequently, any cause, such as varying 
temperature, which sets up even very feeble currents, may so 
retard the ultimate clearing as to indefinitely prevent it. 
If the vessel be kept in the quiet and with as nearly constant 
temperature as is possible, the liquid finally becomes uniformly 
opalescent throughout. This opalescence gradually fades for 
atime, from the subsidence of some of the suspended matter, 
and if the water contains a sufficient amount (which may be a 
very minute proportion) of certain salts in solution, it becomes 
practically clear, that is, clear to the unaided eye in ordinary 
aylight,—as clear as the clearest natural waters. If, however, 
the water be free from dissolved substances (other than the 
clay itself) the fading of the opalescence ceases after some time, 
1t may be in a few weeks or it may be only after two or three 
years, and after that the liquid becomes no clearer by standing. 
The degree of opalescence may and often does vary with vary- 
ing temperature, as if some of the sediment were again picked 
up, or as if a portion which had fallen and remained as a 
mobile liquid stratum on the bottom at one temperature were 
numerous substances not classed as salts or acids), the behavior 
of suspended clays is very different. If a small quantity of 
mineral acid, or some saline substance be added to mudd 
Water, the strata described are either not formed at all, or are’ 
fewer and settle more quickly. If the quantity of dissolved 
material is sufficient, the clay curdles or flocculates and imme- 
diately falls to the bottom. If now the clear saline (or acid) 
