i8o Movement of Microscopic Particles [April, 
solutions of still less power may be mentioned common salt, 
sulphate of potash, iodide of potassium, nitrate of potash, 
cyanide of potassium, ferrocyanide of potassium, lime- 
water, &c. My recorded observations amount to nearly 
eight hundred, and the solutions named were tried not only 
in different strengths, varying according to circumstances, 
from one part in ten to one part in a million, but they were 
tried with various suspended powders, such as charcoal, red 
oxide of iron, amorphous phosphorus, precipitated carbonate 
of lime, red oxide of lead, black oxide of manganese, and 
occasionally with other substances. I do not think, then, 
that I can he much mistaken in my chief conclusions. 
While salts and substances dissolved in water prevent 
pedesis, as a general rule, there are certain remarkable ex- 
ceptions of great significance. One of these is pure caustic 
ammonia, the liquor ammonise of the apothecaries. One 
per cent of ammonia will be found to have little or no effeCt 
in precipitating powders from water. The compounds of 
ammonia are quite different from ammonia itself, and, so far 
as I have observed, all have precipitating power, though 
apparently less than that of most other salts. Other very 
notable exceptions are boracic acid and silicate of soda. 
Upon these, especially the latter, I have tried many experi- 
ments. With i per cent solutions of these substances clay 
was observed to remain in suspension for twenty-four hours, 
and in some experiments a slight cloud of clay remained 
after five days. Indeed I believe that silicate of soda has a 
positive retarding power, and prolongs the suspension of fine 
particles. This salt will also neutralise the precipitating 
power of the acids : when clay is mixed, for instance, with 
water containing one per cent of silicate of soda and one- 
tenth per cent of nitric acid, the acid combines with the 
soda, liberating silicic acid. Now soluble silicic acid has 
no perceptible power in preventing pedesis, as proved by 
experiments upon two samples of very pure silicic acid, 
carefully prepared for me by Prof. Schorlemmer, of Owens 
College, for which I am much indebted to him. Clay, mixed 
even with a concentrated solution oi pure silicic acid, will show 
pedetic motion of minute particles in the microscope. 
One of the most extraordinary faCts connected with this 
subject is the power which gum arabic possesses of increasing 
the pedetic motion. The more inactive substances, such as 
powdered galena, oxide of iron, or carbonate of lime, when 
examined with a weak pure solution of gum arabic (say 
5 per cent), exhibit much motion, and there is the corre- 
sponding absence of precipitating power, or rather a power 
