Such
determinations, however, are seldom required; and from a practical
point of view this combined water causes no trouble.
_In assaying ores_, we term "moisture" all water which is lost by
exposure in a water-oven at 100° C., and the "dry ore" is the ore which
has been dried at this temperature. No advantage, but rather endless
confusion, would be caused by varying the temperature with the object of
estimating the whole of the water which a hydrated salt may contain. The
results of the assay of the other components should be calculated on the
"dry ore." One advantage of this is obvious:--The dry ore has a constant
composition, and the results of all assays of it will be the same, no
matter when made; the moisture, however, may vary from day to day, and
would be influenced by a passing shower of rain. It is well to limit
this variability to the moisture by considering it apart, and thus avoid
having the percentage, say, of copper rising and falling under the
influence of the weather.
In the case of certain salts, however, such as soda crystals and
hydrated sulphate of copper (when these constitute the bulk of the
substance to be assayed), it is as well to perform the assay on the
moist, or at any rate air-dried, substance.[2] It would be equally
convenient to calculate on the substance dried at 100° C.
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