1878.] 
179 
Suspended in Liquids. 
and vibratory, and quite indeterminate in direction, and that 
any one jump of a particle probably never exceeds i-ioooth 
part of an inch at the most, it becomes exceedingly un- 
likely that any particular particle should make ten such 
steps in the same direction. According to the theory of 
probability the proportion of particles which would succeed 
in moving any perceptible distance would be imperceptibly 
small. 
Proceeding with experiments upon many solutions and 
liquids, this faCt came out by degrees more and more 
clearly — that it is pure water which exhibits pedesis in the 
highest perfection. So remarkably is this the case that ike 
suspension of china clay may he used as a test of the purity 
of water. Even the air and carbonic acid, which is usually 
dissolved in water, produces a perceptible difference. If a 
milky mixture of china clay and distilled water be divided 
into two portions — and while one is well-boiled in a glass 
tube and then corked up, the other is shaken in a large 
bottle with air, and then left to stand in a similar glass tube 
— the clay will be found to remain longer in suspension in 
the boiled water. In a similar way, if china clay be mixed 
with various specimens of water, — such as spring water, 
river-water, rain-water, boiled rain-water, — it is possible to 
deteCt differences in the rate of precipitation. In experi- 
menting upon such mixtures it will be noticed that the top- 
most layer of water, to the depth of one-eighth or one- 
quarter of an inch, becomes very clear before the rest of the 
liquid : this may probably be attributed to the air dissolved 
by the water with which it is in contaCt. The quantity of 
air absorbed by water is probably quite competent to pro- 
duce this effeCt, as it often amounts to several hundred- 
thousandths by weight of the water. 
It has been stated that, as a general rule, all substances 
dissolved in water tend to prevent pedesis ; but it is easy to 
deteCt differences in this effeCt. The mineral acids — sul- 
phuric, nitric, or hydrochloric — have an extraordinary effeCt, 
and it is possible to deteCt one part of sulphuric acid in a 
million parts of water by the precipitation of china clay. 
Caustic alkalies and metallic salts have a less, but still a 
great, influence. I have tried the nitrates of copper, mer- 
cury, and silver, the sulphates of copper and zinc, chloride 
of gold, acetate of lead, bichromate of potash. Various 
other acids — such as oxalic, fluoric, hydro-fluosilicic, citric, 
arsenic acids, — also have considerable precipitating power. 
Somewhat lower in the scale may be placed such salts as 
carbonate of potash and soda, chlorate of potash. Among 
