432 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



The movement of the smallest particles in pedesis is always the 

 most active, while in the majority of cases particles greater than the 

 - () 1 0() th of an inch are wholly inactive. A drop of common ink 

 which has been exposed to the air for some weeks, or a. drop of fine 

 clay (such as the prepared kaolin used by photographers), shaken up 

 with water, is recommended by Professor Jevons, 1 who has recently 

 studied this subject, as showing the movement (which he designates 

 pedesis) extremely well. But none of the particles he has examined 

 is so active as those of pumice-stone that has been ground up in an 

 agate mortar ; for these are seen under the microscope to leap and 

 swarm with an incessant quivering movement, so rapid that it is 

 impossible to follow the course of a particle, which probably changes 

 its direction of motion fifteen or twenty times in a second. The 

 distance through which a particle moves at any one bound is usually 

 less than .-.oWyth f an i ncn - This 'Brownian movement' (as it is 

 commonly termed) is not due to evaporation of the liquid, for it 

 continues without the least abatement of energy in a drop of aqueous 

 fluid that is completely surrounded by oil, and is therefore cut off 

 from all possibility of evaporation ; and it has been known to con- 

 tinue for many years in a small quantity of fluid enclosed between 

 two glasses in an air-tight case ; and for the same reason it can 

 scarcely be connected with the chemical change. But the observa- 

 tions of Professor Jevons (loc. cit.) show that it is greatly affected 

 by the admixture of various substances with water, being, for 

 example, increased by a small admixture of gum, while it is checked 

 by an extremely minute admixture of sulphuric acid or of various 

 saline compounds, these (as Professor Jevons points out) being all 

 such as increase the conducting power of water for electricity. The 

 rate of subsidence of finely divided clays or other particles suspended 

 in water thus greatly depends upon the activity of their ' Brownian 

 movement,' tor w r hen this is brought to a stand the particles aggre- 

 gate and sink, so that the liquid clears itself. 2 



Pedetic motion depends on, that is, is affected by 



1. The size of the particles. 



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

 vermilion, 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 hinder it. 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 hydrochloric acid, agglomeration soon occurs. Iron pre- 

 cipitated 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 slow r ly. 



But besides the right appreciation of the nature of pedesis, 

 there is the utmost caution required in the interpretation of the 



1 Quarterly Journal of Micro. Science, N.S. vol. viii. 1878, p. 172. 



2 See also the Rev. J. Delsaulx, ' On the Thermo- dynamic Origin of the Brownian 

 Motions,' in Monthly Journal of Microsc. Sci. vol. xviii. 1877. 



