With this episode FMA made my top 10 favorite anime. It was simply a great episode. I wonder why I didn't pick up on the color of the alchemy from scar's arm before. It was clear that he wasn't an alchemist himself and the arm contained his power, but I hadn't even suspected the power came from the philosopher's stone. It seems the symbol is another form of the stone, just like the red water. Maybe when the power of two incomplete philosopher's stones, made in different ways, interact, they cancel each other out until the weaker one is destroyed. That would explain why scar couldn't use his powers for a few seconds.
I know the accepted english word is alchemy, but shouldn't it be alchemistry?
Maybe when chemistry first seperated itself from alchemy and became a real science, they changed it to seperate it in people's minds from guys trying to turn lead or mercury into gold.