Hull raspberries (remove stems), put into a preserving pan, heat slowly. Do not add any water, but squash some of the fruit near the base of the pan to help break it up. Bring to the boil and simmer gently for about 5 minutes. Add the sugar and heat gently until it has dissolved, then boil rapidly until setting point is reached— about 5 minutes. Pour into hot sterilized jars and cover tightly.