1878.] Suspended in Liquids. 18 1 
of maintaining substances in suspension. It is obviously on 
this account that gum arabic is an invariable component of 
common ink, though the quantity specified in various recipes 
ranges from about 1 to 5 percent. The extraordinary pedesis 
seen in old ink is due to the gum arabic, and perhaps also 
that seen in gamboge. 
It will not be difficult now to arrive at an explanation of 
the pedetic motion. When we compare the substances 
which do not prevent the motion with those which do, it be- 
comes apparent that, with some doubtful exceptions, they 
differ widely in the power of making water a conductor of 
electricity. The following is a quotation from one of Fara- 
day’s earlier researches :* — ££ Some acids, as the sulphuric, 
phosphoric, oxalic, and nitric, increase the (conducting) 
power of water enormously, whilst others, as the tartaric 
and citric acids, give but little power ; and others, again, as 
the acetic and boracic acids, do not produce a change sen- 
sible to the voltameter. Ammonia produces no effect, but 
its carbonate does. Sulphate of soda, nitric, and many 
soluble salts produce much effect. Percyanide of mercury 
and corrosive sublimate produce no effect ; nor does iodine, 
gum, or sugar ; the test being a voltameter.” 
The argument in the case of pedesis is exactly analogous 
to that which Faraday employed in his inquiry into the pro- 
duction of electricity by Sir W. Armstrong’s electrical boiler. 
Faraday found that the machine must be supplied with pure 
distilled water in order to yield much electricity. The 
smallest drop of sulphuric acid, or a little crystal of sulphate 
of soda, dissolved in the water, prevented the evolution of 
electricity. ££ So also did the addition of any of these saline 
or other substances which give conducting power to water.” 
Then Faraday argued in this way : — ££ As ammonia increases 
the conducting power of water only in a small degree, I 
concluded that it would not take away the power of excite- 
ment. Accordingly, on introducing some to the pure water 
in the globe, electricity was still evolved. But the addition 
of a very small portion of dilute sulphuric acid, by forming 
sulphate of ammonia, took away all power. ”t Even com- 
mon town’s water was found by Faraday to be unsuitable to 
the production of electricity. The analogy of these circum- 
stances to those of pedesis is so remarkable that little doubt 
can be entertained that the same explanation applies. It is 
perfectly pure water which produces electricity and pedesis. 
* Experimental Researches in Electricity, vol. i., p. 432, § 1355 ; see also 
p. 161, § 554. 
f Ibid., vol. ii. , pp. no, in, § 2090 to 2094. 
