Field of Science

Tin tears

Tin cries. Not tears, but when tin is bent it elicits a peculiar sound called by metallurgists a "tin cry". Indium also lets out a scream when deformed, as Michael Cassidy pointed out in an earlier comment.

You are hearing the sound made by a phase transition, a change in the structure of the metal at the atomic level. Indium in its crystalline form is tetragonal, when bent, the mechanical stress induces "twinning" in which sections of the crystal become mirror images of adjacent planes. Twinning plays a role in mechanical failure of metals subject to stress, the research literature goes back roughly a century.

Listen to a recording of indium "screaming" made by Theodore Gray at the WGBH studios. It's an unnerving sound, more like a crackling than a scream.

The photo is courtesy of David Hammon in the physics department at the University of Vermont.

Silver linings

Yesterday I had a round of minor surgery. When all was said and done, the surgical site was cauterized with what a nineteenth century physician would have called "lunar caustic" -- silver nitrate to a modern chemist or physician. I have to admit my first geeky thought was, "how does that work?"

Silver nitrate has been used for a long time as a cauterizing agent. In 1826 John Higginbottom, a British physician wrote An essay on the application of the lunar caustic in the cure of certain wounds and ulcers. My physician used a solution of AgNO3, Higginbottom almost certainly used a solid mixture of silver nitrate and silver chloride, but other than that the basic treatment protocol hasn't changed in almost 200 years. Higginbottom notes that the application "smarts" and I would guess that it must have. I felt it, even with a good local anesthetic. The good 19th physician also prescribed adjuvant therapy ("I took away ten ounces of blood and administered purgative medicine") which my physician sensibly eschews!

So why is the stuff called lunar caustic? The caustic part is obvious, silver nitrate is an effective oxidizing agent for organic molecules, including biomolecules. Alchemists associated silver with the moon, its Latin name, argentum derives from "white, shining".