From 1922:

Take 1 lb, 6 oz. castor sugar, 1 oz. gelatine, a small saltspoonful tartaric acid, 1 lemon, a tablespoonful peppermint essence, a few drops green coloring.
Dissolve the gelatine in three-quarters of a pint of cold water, put it in a saucepan with the castor sugar, and stir over gentle heat. When the sugar has dissolved, boil rather quickly for ten minutes. Take off the fire, stir in the tartaric acid and strained lemon juice; add the green coloring, just bring to the boil again, take off fire, and add the peppermint when the mixture is getting cool. Pour into an oiled or wet dish. When set, turn on to icing sugar, cover with icing sugar and leave for two or three hours.