In a preserving pan over medium heat, mash the raspberries with a potato masher. Add the sugar and lemon juice. Continue to heat the mixture, stirring gently, until the sugar is dissolved. Raise the heat to medium-high, and boil the mixture, stirring frequently, to 218 degrees F. or until a drop of the jam mounds in a chilled dish. This will take about 6 to 7 minutes.