When the experiment is made very carefully in a proper way this material can be separated from the pure clay. It is called silt, but really there are a number of silts, some almost like clay and some almost like sand; they shade one into the other.
If there is enough grit it should be weighed: we obtained 14 decigrams of grit from 10 grams of our top soil and 17 decigrams from 10 grams of bottom soil. We cannot separate the clay from the silt, but when this is done in careful experiments it is found that the subsoil contains more clay than the top soil. We should of course expect this because we have found that the subsoil is more sticky than the top soil. These results are put into the columns as before so that we can now see at once how much of our soil is water, how much can burn away, how much is grit, and how much is clay and other things.
What would have happened if the sample had been dug out during wetter or drier weather? The quantity of water would have been different, but in other respects the soil would have remained the same. It is therefore best to avoid the changes in the amount of water by working always with 10 grams of dried soil. The results we obtained were:—
Top soil Subsoil
Weight of dry soil before burning ... 100 100 decigrams
" " " after " ... 92 97 "
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The part that burned away weighed ... 8 3
Weight of grit from 10 grams of dried soil 17 19 "
The columns are given in Fig. 3.
Fig. 3. Columns showing what 100 parts of dried soil and subsoil were made of
Summary. The experiments made so far have taught us these facts:—