[chef] Adding recipes from to a nodes run_list

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Hi folks!
I'm using chef 0.7.16 and want to add a specific recipe to a nodes run_list
like I can do by using the WebUI, but by writing my own script.
I loaded a cookbook and tought about adding it to the run_list. Of cause that
doesn't work, so I tried to get the single recipes of that cookbook by using
  
  recipes = cookbook.recipes
  recipes.each do |recipe|
    node.run_list << recipe
  end

But after I've done that, the recipes don't apper in the runlist shown in the
WebUI.
What am I doing wrong? Is that maybe the wrong way?

Thanks in advance,
Kris