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16 any thing of importance to the soap-boiler.

Alkalis are of two kinds, fixed and volatile. The fixed kind are subdivided into two, the vegetable, and mineral or fossile alkali. The vegetable is so called, because it is procured from the ashes of burnt vegetables; the fossile, because it is found native in some places of the earth, and is the basis of sea-salt, and, in other parts, is dug out of mines in vast quantities.

They are called fixed, because they endure a very intense degree of heat without being dissipated in vapour, so as even to form a part of the composition of glass.

The volatile alkali is generally obtained by distillation from animal substances. In its pure state, this alkali is perfectly invisible, but affects the sense of smelling to such a degree as not to be approached with safety.