Bukkit

I'm having a couple problems with particle effects...

Firstly, the particles will only spawn if the grenade actually hits an entity, rather than whenever it blows up.
Secondly, the "gravity" arg doesn't appear to do anything at all.

Here is what I have set up for my particle effect:
            Sparkle:
                type: particle
                args:
                - type: flame
                - amount: 1
                - gravity: 1000000
                - max-age: 600
                - scale: 3
                target:
                    type: Targetlocation
                    args:
                    - radius: 3