how to add a css class to a Raphael object

I'm trying to create a webpage which using a lot of Raphael objects like lines, rectangles, circle. I'm using different colors for each of the event on these objects like onmouseover one color, onmouseout another etc.. So since I have a lot of styling I was wondering if I can specify a css class to these objects. I tried using the following code on IE, but I could not see the styling effect

rectObj.attr('class','mediumBold');

mediumBold is one the defined css class.

I'm fairly new to this. Any pointer will be helpful.

Thanks.