This section is from the book "The Relation Of Food To Health And Premature Death", by Geo. H. Townsend, Felix J. Levy, Geo. Clinton Crandall. Also available from Amazon: Clean Food: A Seasonal Guide to Eating Close to the Source with More Than 200 Recipes for a Healthy and Sustainable You.
"The lemon has long been used for its flavor, and in a medicinal way, but modern chemistry so perfectly counterfeits all flavors that the use of fruits for such purposes is almost discontinued, so that the lemon must hold its place for its valuable acids."
"Citric and malic acids. A lemon does not contain any properties that could really be called a food, and its use is really only that of a cleanser. It is especially valuable to cleanse the stomach of mucus, when its juice is used with hot water an hour before meals. No sugar should be used. As a toilet article for the skin, hair, and mouth, it has no equal, for its juice cleanses the skin of an excess of fat, and should be used to take away the "shine" on the face, the ladies so much dread, instead of face powders. It will also remove blackheads, due to impaired circulation of the skin, and is truly nature's beautifier. The juice of the lemon when used without soap is an invaluable hair wash to remove dandruff and oil, and will also cleanse and sweeten the mouth, when there is a bad, or 'dark brown' taste. For washing the hair, the juice of a fresh ripe lemon should be squeezed into a pint and a half or two pints of lukewarm water, and thoroughly rubbed into the scalp, then dried with a rough towel."
 
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