Supposing the last of these preliminary distillations to have been completed, the residues left in the retorts are removed, and the retorts, as they lie in the hot furnace, are charged by mean: of semi-cylindrical shovels, and their adapters put on.
On distilling such a mixture pure A will come over first, followed by mixtures in which the quantity of B continually increases; consequently by a sufficient number of distillations A and B can be completely separated.
For dry distillations the retorts are generally horizontal cylinders, the bottom or lower surface being sometimes flattened.
Of artificial productions the most fruitful and important is provided by the destructive or dry distillation of many organic substances; familiar examples are the distillation of coal, which yields ordinary lighting gas, composed of gaseous hydrocarbons, and also coal tar, which, on subsequent fractional distillations, yields many liquid and solid hydrocarbons, all of high industrial value.
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