It’s hard to find an odor beginning with X, but this particular substance figured large in my graduate career. Xylene, from the Greek word xylos, meaning wood, is an aromatic hydrocarbon. I won’t bore you with the chemical formula, and just because xylene is an aromatic hydrocarbon doesn’t mean it smells good. It just means it contains one or more benzene rings.
Xylene is highly flammable, can irritate the eyes and should not be inhaled, although if you are working in a histology lab, this sometimes can’t be avoided. Hence, I know its smell. This chemical is used in the leather, rubber and printing industries because it is a solvent, i.e., other chemicals dissolve easily in it. It’s also an ingredient in paints, lacquers, varnishes, inks, dyes, adhesives and cleaning fluids, and is used as an industrial de-greaser and in motor and aviation gasoline blending agents.
My exposure to xylene came in my undergraduate and graduate histology courses. Here is a very simplistic explanation of where xylene comes into the processing of tissue for histological examination. After a piece of tissue is fixed (a chemical process that kills the tissue so postmortem decay is prevented and the tissue is preserved), it is dehydrated in alcohol, and then the alcohol has to be removed from the tissue. This is where xylene comes in, as a transitioning agent: it removes alcohol from the tissue, along with any lipids, rendering the tissue clear. The tissue is then infiltrated with an embedding medium so that it can be sectioned for examination. In truth, this process is very complex and there are many different organic compounds that can be used along the way, depending on the type of tissue being processed and what the histologist wants to preserve in that tissue.
I rarely ran histological preps after those courses, but I remember the long hours spent in the histology lab and the various smells associated with the different steps in the preparation process. And guess what? Now they have machines to do it!