NOTES AND MEMORANDA. 
273 
opposite character to what Professor Barrett anticipated. With a 
solution of common soap the pedetic motion becomes considerably 
more marked than before. I have observed this result not only with 
china clay and some other silicates, but also with such comparatively 
inert substances as the red oxide of iron, chalk, and even the heavy 
powder of barium carbonate. The last-named substance, one of those 
which we should least expect to dance about of its own accord, gave 
a beautiful exhibition of the movement when mixed with a solution 
of about 1 per cent, of soap, and viewed with a magnifying power of 
500 or 1000 diameters. 
The correctness of this result was also tested by observing the 
suspending power of solutions of soap-solution compared with water. 
If a little china clay be diffused through common impure water, 
that, for instance, of the London Water Companies, the greater part 
of the clay will soon be seen to collect together in small flocks and 
fall to the bottom in two or three hours, the water being almost clear. 
However, if about 1 per cent, be dissolved in the water, the behaviour 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 common 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 the 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 suspen- 
sion, two or three days at least. 
These facts 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 alkaline salts be added, dissolved' in water, it becomes capable of 
acting upon oleaginous matter, but the pedetic power is lessened, not 
increased. But if the soap be added also, we have the advantage 
both of the alkali 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 universal 
substances which increase the pedetic and suspensive power of 
water. 
I believe that the detergent power of soap and water is one of 
