The pressure to preserve

Stephen Davey, associate editor for Nature Chemistry, blogged at the Sceptical Chymist about visiting the National Archives and seeing the Declaration of Independence, the Constitution and the Bill of Rights. He was surprised to find that the documents were stored under helium as opposed to argon - and wondered why. That started me wondering as well, particularly since the inert gases are not interchangeable in all circumstances (you can use helium to dilute the air mixture for diving, but not argon, for example.)

Helium is both more expensive (not an issue in this context, the cost of the gas inside the cases has got to be the least expensive piece!) and difficult to work with than argon. It can leak out through materials that seem air and water "tight". That's why those latex balloons that looked so cheery on the day of the party are withered and droopy by the morning. They're waterproof, but not helium proof.

In the 1950s the US National Bureau of Standards (now NIST) was charged with deciding on the best way to preserve the Charters of Freedom (the three founding documents of the United States of America). (You can read the full report here.) Helium was chosen, despite its propensity to leak through many materials, partly because a high purity, local source was readily available but most because of its thermal conductivity.

The designers of the encasements wanted a way to measure the pressure of the helium within the cases without having to open them, or remove a sample. Since the thermal conductivity of helium is very different than that of air, changes in the thermal conductivity (how heat moves between the panes) could be used to detect leaks. Argon's thermal conductivity is similar to air, so if argon leaked out and air in, the change would be hard to detect.

New casements were designed about ten years ago, with argon as the gas of choice this time. Sapphire ports are embedded to allow the atmosphere inside the cases to be monitored spectroscopically - by passing a beam of light through the port. Since the new methods of monitoring don't require the inert atmosphere to have a different thermal conductivity, it allows argon - which can't wiggle its way out the way helium can - to be used.

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The photo is from The Science News-Letter, vol. 62 (Dec. 6 1952), p. 359.

Word Wraps: From the ACS meeting

I am at the ACS meeting in Washington DC, here as "press" rather than chemist. It's a very different way to see the meeting. I went to a press briefing this morning - on the first phases of development of aresol vaccines for measles (Robert Sievers). The press center is tucked away next to the registration, and has everything a writer might want: food, wireless access and a steady stream of caffeine and conversation.

The briefings are being streamed live on the web and journalists watching can send their questions in to be asked. Miss something the first time round? Watch the replay here.

Listening as a scientist to a talk, and as a writer to the briefing turn out to be slightly different experiences. Both require critical listening, but listening as a writer prompts me to think far more about the words the science is coming wrapped in. The shorthand scientists use sounds almost staccato in this context. "Measles naive" instead of "never exposed to the measles virus" or "no evidence of viremia" instead of "no measurable virus in the bloodstream".

We try to be both precise and concise, but I wonder how often the combination in giving a talk, or even reading a paper in the literature leads to attentional processing deficits? An interesting experiment in attentional processing is to present subjects with a rapidly changing sequences of letter, interspersed with numbers. If two numbers are placed too close together, subjects can "miss" the second letter while their brain is busy processing the first. Pack too much into a sentence, and your "subjects" might miss bits.

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My Thesis column in Nature Chemistry this month, Stretching Toplogy, takes a slightly different tack in thinking about the ways words wrap around science.