yet advanced is that based on the emulsifying
power of soap solutions. The fact that these will readily form emulsions
with oils has long been known, and the detergent action of soap has
frequently been attributed to it, the explanation given being that the
alkali set free by the water emulsifies the fatty matter always adhering
to dirt, and carries it away in suspension with the other impurities.
Experiments by Hillyer (_loc. cit._) show, however, that while N/10
solution of alkali will readily emulsify a cotton-seed oil containing
free acidity, no emulsion is produced with an oil from which all the
acidity has been removed, or with kerosene, whereas a N/10 solution of
sodium oleate will readily give an emulsion with either, thus proving
that the emulsification is due to the soap itself, and not to the
alkali.

Plateau (_Pogg. Ann._, 141, 44) and Quincke (_Wiedmann's. Ann._, 35,
592) have made very complete researches on the emulsification and
foaming of liquids and on the formation of bubbles. The former considers
that there are two properties of a liquid which play an important part
in the phenomenon, (1) it must have considerable viscosity, and (2) its
surface tension must be low. Quincke holds similar views, but considers
that no pure liquid will foam.

Soap solution admirably fulfils Plateau's second condition, its surface
tension being only about 40 per cent. of that of water, while its
cohesion is also very small; and it is doubtless to this property that
its emulsifying power is chiefly due. So far as viscosity is concerned,
this can have but little influence, for a 1 per cent. solution of sodium
oleate, which has a viscosity very little different from that of pure
water, is an excellent emulsifying agent.

Hillyer, to whose work reference has already been made, investigated the
whole question of detergent action very exhaustively, and, as the result
of a very large number of experiments, concludes that the cleansing
power of soap is largely or entirely to be explained by the power which
it has of emulsifying oily substances, of wetting and penetrating into
oily textures, and of lubricating texture and impurities so that these
may be removed easily. It is thought that all these properties may be
explained by taking into account the low cohesion of the soap solutions,
and their strong attraction or affinity to oily matter, which together
cause the low surface tension between soap solution and oil.

CH