A friend has asked the following question,  anyone know what to advise on
the question?

 I was wondering if you might help me answer a relatively simple question
 on histology technique.  I've got a protocol that is for staining and
 dehydrating nissl sections for brain tissue.  It goes through several
 steps, starting by dehydrating and then soaking in 50/50 CHCl4/100%EtOH,
 then hydrating to stain, then clearing, and dehydrating and finally
 soaking in xylene before coverslipping.

What are the purposes of using  chloroform and xylene in clearing/staining
tissues?  Do they serve  different purposes?  Do you know what they actually
do to the tissue, chemically?  Any help you could give me would be wonderful
and  informative.