Mix the treacle, sugar, butter and vinegar in a saucepan, and boil without stirring until a few drops in cold water are brittle. Add the bicarbonate of soda to the mixture. Boil again for about eight minutes, pour out on to a greased tin, and sprinkle the lemon juice on top. When it is cool enough to handle, smear a scrap of butter on the hands and roll the toffee into small balls. Leave to harden.