SEA SAND. 
51 
Since Fig. 1 will do equally well for large shot, seen at a 
distance, or for small shot seen through a magnifying glass, 
we may argue that at any rate the interspaces are the same 
fraction of the whole in either case. But if the pattern of the 
packing is different as in Fig. 2, and the spheres are closer 
together, a mathematical inquiry should show what different 
volumes of water could he contained in each case. But even 
on the tightest packing there is room for very small particles 
between the large ones, and therefore we have an a priori 
suspicion that a sand of mixed grades will fill space more 
completely than another of uniform grain. 
Experimentally we find that shot whether larger (2*8 mm. 
diam.) or smaller (1*9 mm. diam.) will really fill about 62% of 
the space which it occupies, leaving 38% of the space 
which may he filled with water, the size of shot making no 
difference, provided they are all of the same size. 
The Thornwick Bay sand—a coarse very angular sand of 
even grade—gave results very nearly the same; 7 experiments 
giving figures between 36 and 39, and thus suggesting that the 
shape of the sand makes hut little difference to the interspaces. 
The coarse Bedfordshire sand used by gardeners has rounded 
particles which differ considerably in size. This contains 
much less water, the answers lying between 29 and 33, and 
averaging 31 per cent. This bears out the suggestion that 
in a sand of uneven grade, the finer particles would fill up the 
spaces between the larger. 
Wh en wet sand is allowed to drain, some, but not all, of 
the water runs away. Our next experiment with funnel and 
graduated jar is to measure the water detained by the sand, 
and here we find a curious difference from the figures just 
quoted. The Thornwick Bay sand which contains the more 
retains the less on draining, and the Bedfordshire which can 
only contain 31 % seems able to detain it all. 
This brings us to another property evidenced by the sand— 
the retaining power which narrow spaces have for water. This 
is called capillarity, and may be illustrated by the power of 
blotting paper to absorb ink. In Germany fine sand is often 
used instead of blotting paper. 
