This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Vegetable substances, burnt in the open air, are resolved, partly into smoke, which, con-densed, forms foot; and partly into white ashes, which generally give out, on being boiled in water, a fixt alkaline salt. Animal substances, and mineral bitumens, are resolved in like manner into foot and ashes; with this difference, that the ashes of these yield rarely any alkaline matter, and that they emit in burning a different kind of smell.
On submitting the same subjects to a like degree of heat in close vessels, different products are obtained. From most vegetables there arises a watery and acid liquor; a reddish, empyreu-matic, acrimonious oil, which swims on its fur-face; at length, in the utmost degree of fire, a thicker black oil, which sinks to the bottom; and sometimes a little volatile alkaline salt: from animals, a watery and alkaline liquor, a volatile alkaline salt, and oils of a more fetid kind: from bitumens, an acidulous liquor, an oil approaching to the nature of petroleum, and sometimes a concrete subacid salt: a black infi-pid coal remaining in all cafes behind. About the appearance of the firstt oil, there is commonly extricated a large quantity of air, or elastic vapour, which, if the fire is hastily urged, and no exit allowed it, either bursts the vessels, or blows off the receiver.