Fade In and Out two images [JavaScript]

I'm trying to make a simple slider using 2 buttons and fade effect. Everything should be working but it's not. What's wrong with my code?

function hideFirst()
{
        $("#SliderBack1").fadeIn('fast');
        $("#SliderBack2").fadeOut('fast');
}

function hideSecond()
{
        $("#SliderBack2").fadeIn('fast');
        $("#SliderBack1").fadeOut('fast');
}

<div id="Slider">
                <img id="SliderBack1" src="Files/Images/slider1.jpg" />
            <img id="SliderBack2" src="Files/Images/slider2.jpg" />
        </div>