2.52 ◊ 5 = 12.6 ~12.6%.~

There are other ores which are not apparently wet, but in the state
called "air-dried." It is easier to take fair samples of these, and,
consequently, it is not necessary to use so large a quantity as 20
grams. But with a smaller quantity, extra precautions must be taken. All
dry solids at ordinary temperatures absorb moisture from the air. The
amount varies with the nature of the material and with the quantity of
surface exposed. Light bulky powders absorb more than heavy ones,
because of the greater condensing surface. It is on this account that it
is well to weigh substances, which have been dried, between
close-fitting watch-glasses. The method of determining moisture is to
weigh out into the glasses 5 grams of ore, and dry in the water-oven
until there is no further loss of weight. On taking the glasses out of
the oven, they should be at once closed, the clip put on, and after
cooling in a desiccator weighed. If after a second trial the loss is the
same, or only increased by a milligram, the determination is finished.

Example:--

Weight of glasses + pyrites 31.0470 grms.
" " glasses 26.0470 "
-------
" " pyrites 5.0000 "
" " glasses + pyrites, dried 1 hour 30.8965 "
" " " " dried 1-1/2 " 30.8957 "
" " " " 31.0470 "
" " " " dried 30.8957 "
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