Alkalies, in chemistry, signify those substances which possess the following properties : viz. they are 1. incombustible; 2. capable of converting a vegetabable blue to a green colour; 3. they manifest a hot and caustic taste; and 4. are soluble in water.

Alkalies are divided into two kinds, fixed and volatile. The fixed are subdivided into vegetable and mineral; the former being the production of burnt vegetables in the open air; and the latter have sometimes been found native in the earth, though we generally obtain our soda by the calcination of marine plants, chiefly from the different species of the glass-wort, or Salsola, L. as well as from other saline vegetables growing near the sea-shore. — See the article Barilla.

Both the fixed alkalies endure a very intense degree of heat, with-out dissipation, and are used in the composition of glass : the volatile are produced by distillation from animal substances ; in their pure state they are invisible, and so pungent to the smell, that they cannot be approached without great danger.

Ail vegetable substances contain fixed alkali, in greater or less proportion. M. M. Deyeux and V au-QUELIN have proved by recent experiments, that one pound of the ashes of horse-chesnuts yields nearly six ounces and a half of pot-ash ; nay, the same quantity of the burnt husks produced more than six ounces. But, according to an accurate analysis made by these chemists, the greatest quantity of vegetable alkali is contained in the fruit of the Spanish lilac, or sy-ringa vulgaris, L. the ashes of which yield more than one-half of pure alkali, or in proportion of eight ounces and three drachms a pound.

M. JacobsoN, the editor of the technologicical Dictionary, asserts, that the thy or withered leaves of the beech-free, or the Fagus sylvatica L. afford the vege-table alkali in great abundance, insomuch that ten pounds weight of the ashes thence obtained, are equal to thirty pounds of common wood-ashes.—We have purposely mentioned the results of these experiments, as the vegetables alluded to may be readily procured, and substituted for the very expensive arti-cles of pearl-ashes and soap. A farther account of useful substitutes will appear under the different heads of Soap, Soda, and Washing.

It is affirmed, that pestilential fluids are rendered harmless and inactive by alkaline substances; and Dr. Mitchill, of New-York, in two letters written to a young lady, has ingeniously and humorously described their good effects. As these refer to many articles of domestic economy, which are more or less "composed of alkaline productions, we shall present our readers with an extract, nearly in the author's own words:—It is a stale and indelicate subject of jesting among men, how much time and labour are consumed by women in scrubbing, scowering, whitening, and washing. These operations, however, are not performed for mere pleasure, but to prevent the conversion of impurities to infection; or to destroy i already produced. For this purpose, they employ pot-ash and its soap, lime, calcareous earth, etc. to scower the porous materials of (heir floors and stair-cases; to purify garments that have become soiled or contaminated by long use, of wearing; and with good reason these substances are capable of drawing forth and rendering harmless, those animal exhalations which are ready to be converted into pestilential poison.—The ladies have indeed proved from long esta-ted experience, that "infection is uniformly prevented and extinguished by the use of alkalies."

Dr. M itchill also recommends the use of pot-ash cakes for children, to prevent the injurious effects of an acid upon their stomach, and mitigate the disorders to which their bowels are liable. He observes, that those infants who have been accustomed to eat cakes a little tinctured with this excellent ingredient, grow fat and healthy ; and concludes with advising alka-lrne washes and powders as dentifrices : which, in his opinion, have been beneficial only in proportion to the alkali, of which they are partly composed.