Salad Dressing That Will Keep

From 1913:

One-third of a pint of good vinegar, two thirds of a pint of water. Bring these to the boil, having ready the following mixture: Two teaspoonfuls of mustard, one teaspoonful of salt, four tablespoonfuls of flour, five tablespoonfuls of sugar. Mix these ingredients well, add a little water and the beaten whites of two eggs. Stir well and pour in gradually the hot vinegar and water. Boil until thick, stirring all the time. Put into glass jars, and when it is cold cover with a close lid.