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The amount and weight of this moisture varies with the humidity of the atmosphere, and the latter may change from hour to hour. Hot objects, such as ignited crucibles, should be allowed to cool in the air until, when held near the skin, but little heat is noticeable. If this precaution is not taken, the air within the desiccator is strongly heated and expands before the desiccator is covered.
Pumice moistened with concentrated sulphuric acid may be used in place of the calcium chloride, and is essential in special cases; but for most purposes the calcium chloride, if renewed occasionally and not allowed to cake together, is practically efficient and does not slop about when the desiccator is moved. Desiccators should never remain uncovered for any length of time.
PROCEDURE. Spread 25 grams of the powdered sample on a weighed watch-glass; weigh to the nearest 10 milligrams only and heat at 105°C.; weigh at intervals of an hour, after cooling in a desiccator, until the loss of weight after an hour's heating does not exceed 10 milligrams.
Cool the crucible in a desiccator, and weigh. Heat again for twenty to thirty minutes, cool, and again weigh, repeating this until the weight is constant within 0.0003 gram. The filter is then ready for use. Place the crucible in the funnel, and apply a gentle suction, !after which! the solution to be filtered may be poured in without disturbing the asbestos felt.
When platinum utensils are used the danger of theft should always be kept in mind. All crucibles, of whatever material, must always be cleaned, ignited and allowed to cool in a desiccator before weighing, since all bodies exposed to the air condense on their surfaces a layer of moisture which increases their weight.
!Ignition of the Iron Precipitate! Heat a platinum or porcelain crucible, cool it in a desiccator and weigh, repeating until a constant weight is obtained. Fold the top of the filter paper over the moist precipitate of ferric hydroxide and transfer it cautiously to the crucible.
In all weighings after the first it is well to place the weights upon the balance-pan before removing the crucible from the desiccator.
Therefore, in order to be fair to the buyer who purchases his material by weight, they have in all great silk centres what they call silk-conditioning houses, where they test the goods to find out how much water is in it. This is done by an apparatus known as a desiccator, which tells what the silk would weigh if dry.
Finally set the crucible upright in the triangle, cover it, and heat at the full temperature of a blast lamp or other high temperature burner. Repeat the strong heating until the weight is constant within 0.0003 gram. On this account the weighings must be promptly completed after removal from the desiccator.
At the end of the half-hour remove the thermometer and transfer the porcelain crucible, which now contains sodium carbonate, to a desiccator. When it is cold, transfer the carbonate to a stoppered weighing tube or weighing-bottle.
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