LEMON SPONGE

From 1922:

Two ounces of gelatine, one and three-quarter pints of water, three-quarter pound of pounded sugar, lemon rind and juice. Simmer gently from 10 to 15 minutes, strain and let stand till cold and it begins to stiffen, beat the whites of the eggs and add them to it, and whisk the mixture till it is white. Put it into a mould which has been previously wetted and let it remain perfectly set.