116 Transactions of the Royal Microscopical Society. 
modern unconsolidated clays and muds it is very often difficult 
to completely separate the ultimate particles when the material has 
been dried. The finer granules cohere together and form compound 
larger granules, which might easily be confounded with objects of 
a different nature. It is therefore often desirable to mix up small 
quantities of such material with a little water by means of a small 
and somewhat stiff brush, so as to separate the detached granules 
without breaking up any truly compound grains. A portion of 
this may then be sufficiently diluted with more water, placed on a 
glass slip with a projecting ledge, and covered with a piece of the 
usual thin glass. By this means the larger particles are seen 
separate, but the smaller have a very great tendency to mass 
themselves together by a sort of mutual cohesion. Since the index 
of refraction of the various grains is in all cases considerably greater 
than that of the surrounding water, the outline of even the most 
transparent constituents is well seen, but at the same time this 
difference in refractive power may make it impossible to study the 
internal structure or optical characters of the larger grains. This 
difficulty is overcome by mounting in Canada balsam, which has so 
nearly the same index of refraction as that of many of the consti- 
tuent grains that, even when their outline is as irregular as possible, 
light passes through them almost as though they were thin slices 
with parallel polished surfaces. This enables us to study the 
external staining, internal structure, and optical characters to 
great advantage, since they are not interfered with by any dark 
shading due to the bending of the light out of the line of vision. 
When examined in water there is no difficulty in recognizing 
extremely minute granules of the kaolin of clays, whereas when 
mounted in balsam they may be almost or quite invisible: but 
this very circumstance is of great advantage in observing certain 
facts, since by making them invisible other objects may be dis- 
tinctly seen which otherwise would be completely hid by the sur- 
rounding granules. ; 
Though, in order to obtain a knowledge of the general cha- 
racter of the deposit and the variation in size and relative 
abundance of the different constituents, it is best thus to examine 
deposits as a whole, a mixture of very coarse and very fine par- 
ticles makes it difficult to study in detail either to perfection. It 
is therefore often desirable to separate the coarse or finer sand 
from the still finer granules of clay, which may be effected by sus- 
ending the material in water, and allowing the sand to subside, 
whilst the whole is gently agitated so as to prevent the cohesion of 
the particles into the compound grains described in a subsequent 
part of this address. 
