function showLayer(layerName)
{
	chapters = new Array("ch1","ch2","ch3","ch4","ch5","ch6","ch7","ch8","ch9");
	
	
	// close any open chapters
	for (i=0; i<chapters.length; i++)
	{
		chapter = document.getElementById(chapters[i]);
		
		if (chapter.style.display == "block")
			chapter.style.display = "none";
	}
	
	// open the requested chapter
	document.getElementById(layerName).style.display = "block";
}