ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 77 
As the sol is washed down most of it passes to the permanent , 
water level, where it remains and accumulates, diffusion being 
practically non-existent. During the dry months a certain 
amount of dessication takes place, involving a deposition of 
the sol as a gel: there is also a certain amount of transformation 
of sol to gel through the presence of electrolytes in the ground 
water.* Some of the humus gels become oxidised, some of both 
humus and ferric hydroxide gels change their colloidal properties 
in other ways. When wet weather comes on again it is no longer 
possible for the whole of the deposited gel to change back to a sol. 
Deposition has begun, and the place where this happened serves 
as a seat of further action, 
This view seems more in accordance with the facts than the 
older one, in that it does not involve any unproved assumptions— 
such as reduction of ferric to ferrous iron and presence of ferrous 
iron in the pan. 
Flocculation and Deflocculation. 
Clay possesses well marked colloidal properties. If rubbed 
with water it becomes plastic, sticky and impervious: it shrinks 
on drying and absorbs heat: on moistening, however, the process 
is reversed and there is considerable swelling and evolution of 
heat. The importance of these observations is equally great in 
agriculture and in the ceramic industry. 
Two hypotheses have been put forward to account for plasticity. 
Rohland attributes it to hydrated colloids present in the clay: 
Atterberg to minute flake-like particles which are able to slip 
over one another without difficulty. 
Another property of clay is of great importance to agriculture, 
and has received much attention from agricultural chemists. 
Addition of a trace of electrolyte—acids or salts—to puddled 
clay causes considerable change in properties: the temporary loss 
of plasticity, impermeability and the power of remaining long 
suspended in water without settling; the clay is now said to be 
flocculated. The change can be watched if a small quantity of 
any flocculating substance is added to the turbid liquid obtained 
by shaking clay with water; the minute particles are then seen 
to unite to larger aggregates which settle, leaving the liquid clear. 
There is, however, no permanent change, deflocculation takes 
place, and the original properties return, as soon as the flocculat- 
ing agent is washed away. Alkalis (caustic soda, caustic potash, 
ammonia, and their carbonates) produce the reverse effect: they 
deflocculate clay, intensifying its stickiness and impermeability, 
and causing it to remain suspended in water for long periods. 
These properties are of considerable importance in devising 
schemes of manuring for soils: it is obvious that alkaline sub- 
stances are to be avoided on clay soils, however rich they might 
be in plant nutrients, as they would produce undesirable 
* Ramann lays stress on this, Morison and Sothers do not because their sols 
were yery stable in presence of electrolytes. 
