How to Make It
In a small bowl, sprinkle the gelatin over the water and let stand until softened, about 5 minutes. In a blender, puree 4 cups of the raspberries with 1/2 cup of the sugar. Strain the puree into a bowl through a fine sieve. In a microwave oven, melt the gelatin for 10 seconds on low power. Whisk the gelatin into the raspberry puree.
In a large bowl set over a pot of simmering water, whisk the egg whites with the remaining 1/4 of cup sugar until warm to the touch. Remove the bowl from the heat. Using a hand-held electric mixer, beat the whites at medium-high speed until stiff and glossy. Fold the egg whites into the puree.
In the same bowl, beat the cream until firm. Fold the whipped cream into the raspberry mixture.
In a small bowl, coarsely mash the remaining 1 cup of raspberries. Fold them into the mousse. Scoop the mousse into 8 bowls and refrigerate until set, at least 1 hour or overnight.