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offensive fumes are developed in this operation, it should not be carried on in a room. After the residue has been heated, a black charred mass remains behind. This is to be kept at a dull red heat for some hours, when the carbon unites with the oxygen of the air, and is dissipated in the form of carbonic acid, while the inorganic salts remain behind perfectly white and pure. The ash is dissolved in water, the solution filtered and concentrated by heat, when crystals may be readily obtained in the usual manner.
In endeavouring to obtain crystals for microscopical examination, the concentration should be allowed to proceed to the proper extent, and then a drop of the warm liquid placed on a slide, and covered with thin glass, so that the crystals may be examined in their own mother liquor. The preparation of which fig. 2 is a copy was obtained in this manner, and a similar plan has been pursued in obtaining all the crystals delineated.
Pure urea may be easily obtained by the decomposition of the nitrate or oxalate of urea.
The crystals represented in fig. 1 were made by decomposing pure oxalate of urea with common chalk. An oxalate of lime is found, which is separated by filtration, and the urea remains in solution. From the nitrate, urea may be obtained by adding carbonate of barytes-nitrate of barytes and urea result; the latter may be separated by evaporation to dryness, and extraction with alcohol, which dissolves the urea, and leaves the nitrate of barytes.
For the mode of preparing the nitrate and oxalate of urea, see pages 57 and 59. Pure urea may also be obtained artificially by evaporating cyanate of ammonia to
Fig. 1. Urea obtained from Urine crystallized in its own mother liquid.
Fig. 2. The same examined in the dry way. The spaces seen in the substance of the crystal contain air.
Fig. 3. Small crystals of urea formed in a concentrated solution of natural urea.
Fig. 4. Similar crystals of larger size.
The spots represented in all these crystals are little cavities within them, which are occupied with fluid when moist, but when dry contain air.