Change other symbols than the symbol itself

Hi

I want to change properties for a button named button, wich is a mc, and I use this AS:

*_root.button.setProperty("",_alpha,“30”);

*the actionscript is inserted as an action that will occur when another button is pressed, but nothing happens to *button…

*ergo; I want to be able to change other symbols than the button that is pressed. how do I access them? I haven’t really got it straight regarding _root etc.