74 
James Small 
comparatively large size of the particles which are considered to 
act in the “creaming” process. 
At the same time, even in colloidal solutions microscopic particles 
can be distinguished in some cases. Linder and Picton 1 , for example, 
“recognised four kinds of sols: a, visible in the microscope.” Further 
Taylor {op. cit. p. n) states that: “The upper limit [of colloidal 
heterogeneity] has been fairly definitely established, and lies above 
the limit of microscopic visibility.” He gives 10 /x as this upper 
limit; “particles between these limits (io /x and o-i /x) are termed 
microns in Zsigmondy’s nomenclature.” According to a recent 
report 2 on Ultramicroscopy “the lower limit for the micron is con¬ 
ventionally fixed at -2 /jl and corresponds with the limit of micro¬ 
scopic visibility as determined by Johnston Stoney.” However, in 
order to remove any further ambiguity, the size of the particles 
which are supposed to cream has been given definitely 3 as “very 
small, almost ultramicroscopic 4 (i.e. *2 /i to *8 /x radius), or quite 
ultramicroscopic (i.e. less than -i /x radius).” Brownian movement 
is shown by such particles, “at this stage [o-i /x] the phenomenon 
known as the Brownian movement, just observable at about io -3 
cm. [=io /x], is well developed 5 .” 
Keeping in mind that the “creaming” particles may be any¬ 
thing up to o*8 /x in radius, we may be allowed to readjust the data 
given in the first section of Professor Blackman’s article. As pointed 
out in that paper, Perrin found “that for gamboge particles of 
radius o-2i /jl the concentration was halved for each rise in height 
of 30 /x.” Since a meristem-cell in a root-tip averages 20 /x to 40 /x 
in diameter, this degree of difference in concentration would seem 
to satisfy the requirements of the “creaming” hypothesis. 
One other point may be noted from §1. It is stated that: “As 
is well known, there is no obvious settling of colloidal solutions.” 
Taylor {op. cit. pp. 56-57) gives amongst “the methods which con¬ 
nect the size of the particle with other properties of the sol,” “ (3) 
the velocity of sedimentation ,” and he also gives the Stokes’ Law 
equation as the basis of this method. It is true he adds that: “It 
1 See The Chemistry of Colloids, by W. W. Taylor, p. 58. London. 1920. 
2 B.A. Rep., Cardiff, 1920. Third Report of the Committee on Colloid 
Chemistry, p. 32. 
3 See A Textbook of Botany, J. Small, p. 347. Churchill. London. 1921. 
4 Since Abbe placed the lower limit of microscopic visibility at o-8 ^ to 
o-2 /jl this phraseology is quite justified (see An Introduction to the Physics and 
Chemistry of Colloids, by E. Hatschek, p. 24. London. 1919). 
5 Taylor, op. cit. p. n. 
