ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 611 



seem an absurd notion, but it was a theory first advanced to account 

 for the phenomenon. And as it was first observed by Eobert Brown, 

 when examining the pollen of plants, he had some ground for his sup- 

 position. Buffon attributed it to this cause, and Spallanzani termed 

 the dancing particles " animaletti d'ultimo ordine." It occurs, 

 however, with particles strictly mineral in their constitution, such as 

 quartz, cinnabar, finely divided gold, &c. 



2. Nor does it depend on the material of which the particles are 

 composed, for all substances, if in a sufficiently fine state of division, 

 manifest this motion, (a) They may be conductors, e. g. gold, silver, 

 platinum, (h) They may be non-conductors, e. g. sulphur, gamboge, 

 quartz, (c) They may be absolutely insoluble in water, (d) They 

 may be slowly attacked by water, e. g. quartz, silicates, barium 

 sulphate, (e) They may be good conductors of heat, e. g. the metals 

 above mentioned. (/) Or bad conductors, e. g. sulphur, gamboge. 

 (g) They may be transparent, or (Ji) opaque. 



3. The motion does not depend on the form of the particles. The 

 question of pedesis is very closely connected with that of the settling 

 of finely divided powders in different menstrua. In a paper commu- 

 nicated to the Geological Society of London in 1876, on the settling 

 of mud, the author showed : (1) that finely divided matter does not 

 quickly settle in pure water. (2) That it settles more quickly in 

 hot than in cold water. (2) That the rate of settling does not 

 depend on the density of the solution, for mud settles more quickly 

 in strong than in weak solutions. (4) It does not depend on the 

 chemical action of the liquid on the solid, for sulphur follows the 

 same rule as other substances. (5) It follows the same order as the 

 absorption of heat, when the salt is dissolved, in the solution of which 

 the suspended particles settle. (6) It depends on the agglomeration 

 of the particles : when the particles acquire sufficient size to have no 

 motion, or a very slow one, they settle quickly. This phenomenon 

 is evidently closely allied to pedetic motion, and is to be explained 

 by it. 



Pedetic motion depends on, that is, is affected by : — 



1. Hie size of the particles. Particles more than l-5000th of an inch 

 in diameter do not jerk about suddenly, but are sometimes seen to 

 oscillate slightly. 



2. The specific gravity of the particles. Metals, or particles of ver- 

 milion, of similar size to particles of silica or gamboge, move much 

 more slowly and less frequently. 



3. The nature of the liquid. No liquid stops pedesis ; but liquids 

 which have a chemical action on the substance do. This action may 

 be very slow, still it tends to agglomerate the particles. For instance, 

 barium sulphate, when precipitated from the cold solution, takes a 

 long time to settle ; whereas, when warm and in presence of hydro- 

 chloric acid, agglomeration soon occurs. Iron precipitated as hydrate 

 in presence of salts of ammonium, and mud in salt water, are other 

 instances. The motion does not cease, but the particles adhere 

 together and move very slowly. 



The moving particles may be either liquid or solid; but the 



2 R 2 



