Combine the 1 1/4 cups sugar and unflavoured gelatine in a medium saucepan. Stir in the water and lemon rind. Heat, stirring often, until the mixture comes to a boil, then lower the heat and simmer for 5 minutes.

Remove the saucepan from the heat. Stir in the lemon juice. Strain the mixture into a 13x9x2 inch metal pan. Cool at room temperature for 30 minutes. Stir in the milk until well blended. Freeze the mixture, stirring several times, so that the sherbet freezes evenly, until almost frozen, about 4 hours.

Using an electric mixer beat the egg whites until foamy and double in volume in a small bowl. Beat in the 1/4 cup sugar gradually, until soft peaks form. Spoon the frozen mixture into a chilled large bowl. Beat with an electric mixer until very smooth. Fold in the egg white mixture quickly.

Spoon the sorbet into a 6 cup mold or bowl. Cover with tin foil or plastic wrap. Freeze for at least 6 hours, or overnight. Unmold, or scoop from the bowl it was frozen in. Makes 1 1/2 quarts.