Place 1/4 cup water in a cup and sprinkle gelatin on top. Set aside to soften.
In a bowl, mash raspberries with a whisk or potato masher and then strain into another bowl to remove seeds. You will have about 1 2/3 cups puree. Add sugar and lemon juice and stir until sugar dissolves.
Microwave softened gelatin for 30 seconds on high power. Measure 1/4 cup raspberry puree and set aside. Whisk gelatin into remaining raspberry puree.
Whip cream until stiff. Gently fold in one third of raspberry-gelatin mixture until nearly incorporated. Fold in remaining raspberry mixture in 2 more batches in same manner. Drizzle reserved raspberry puree onto mousse and fold once or twice. (Do not overblend if you want decorative streaks.)
Scrape mousse into a glass bowl, cover and refrigerate until set, about 3 hours. Serve in individual glass bowls or dishes.