1878.] 
The British Association . 
515 
of the clay is quite different. The larger particles soon 
subside, but the smaller ones remain diffused through the 
liquid for a long time, giving it a milky appearance, quite 
different from the flocky and grainy appearance of the com- 
mon water : if 1 per cent of sodium carbonate be dissolved 
in common water, and china clay be mixed therewith, the 
subsidence of the clay is still more rapid, owing, as I have 
explained, to the increase in the eleCtric conductivity of the 
fluid, and the consequent decrease of pedesis. But I now 
find that if soap be added at the same time pedesis is not 
destroyed, but considerably increased, and the clay remains 
a long time in suspension, two or three days at least. 
These fa<5ts give a complete explanation of the detergent 
power of soap. It has long seemed to me unaccountable 
that for cleansing purposes the comparatively neutral soap 
should be better than the alkaline carbonate by itself. We 
are told that the alkali is but feebly combined with the 
stearic or other fatty acids. But why combine it at all if we 
need only the alkaline power of the base ? The faCt is that 
the detergent aCtion of soap is due to pedesis, by which 
minute particles are loosened and diffused through the water, 
so as to be readily carried off. Pure rain or distilled water 
has a high cleansing power, because it produces pedesis in 
a high degree. The hardness of impure water arises from 
the vast decrease of pedesis due to the salts in solution. 
Hence the inferior cleansing power of such water. If alka- 
line salts be dissolved in the water, it becomes capable 
of acting upon oleaginous matter, but the pedetic power is 
lessened, not increased. But if soap be added also, we have 
the advantage both of the alkaline dissolving power and of the 
pedetic cleansing power. At the same time we have a clear 
explanation why silicate of soda is now largely used in 
making soap ; for I have shown, in the paper referred to, 
that silicated soda is one of the few mineral substances 
which increase the pedetic and suspensive power of water. 
I believe that the detergent power of soap and water is 
one of the many important phenomena which may be ex- 
plained by the study of pedesis, and I propose to follow up 
the investigation of this movement in regard to the several 
substances which tend to increase it. 
Mr. Robert Sabine read a paper “ On Motions produced 
by Dilute Acids on some Amalgam Surfaces.” These mo- 
tions result from an alternate play of deoxidation of the 
mercury underneath the acid by electrolysis, due to the 
currents of small floating particles of the positive metal 
2 L 2 
