Roux makes the best thickening for soups which are not clear, using brown or white roux according to the color of the soup. Thin the roux with a little soup, so it will be smooth before adding it to the soup kettle. Roux added to pea, bean, and potato soups prevents their separating.
A thickening of eggs is made as follows: Beat two or three yolks and dilute them with a half a cupful of cream or milk or cold soup. Stir in a few spoonfuls of the hot soup to warm it. Remove the soup from the fire and stir in slowly the egg mixture, return it to the fire to cook the egg, but do not let it boil, or it may curdle.
Clear soups are sometimes thickened by using one teaspoonful of arrowroot to a quart of soup. Mix the arrowroot with a little of the cold soup, turn it into the hot soup, and cook until it becomes clear. A clear soup so thickened may be flavored with sherry.