Letters: March 2023

The articles by Andrea Sella on historic aspects of chemistry are always highlights of Chemistry World. Perhaps it is because I learned my chemistry at a time when many of the items he describes were still in use. Regarding the early discoveries around light polarisation, I was always enchanted by a large crystal of Iceland spar (calcite) in the museum at Canterbury. Two images produced from one object seemed like magic. A few years ago I found an old book The Prospectors Handbook, no doubt an essential guide to prospectors seeking to make a fortune in the New World. As well as having tables to calculate the horsepower of water wheels, it introduced me to the dichroscope for detecting chirality in minerals and gemstones. Based on a calcite crystal, it is a very handy gadget for discriminating between gems and paste, glass being anisotropic.

Also in the prospector’s toolkit was a direct vision spectroscope. I have several, my favourite being labelled ‘Rain Band Spectroscope’. It was an aid to weather forecasting before the days of national weather forecasts. It worked by the examination of weak absorption bands, caused by water vapour, in the red part of the spectrum. I use it primarily to detect lanthanide elements in glass. Specimens containing neodymium exhibit striking colour changes when examined with white light from different sources. Armed with a few historical chemistry tools, the world of antiques is ideal for the scientifically inclined to seek out treasures which are invisible to the casual observer.

Perhaps Sella could feature the etalon filter in a future article. It enables solar prominence to be seen by viewing the solar disc in the H alpha spectral line. Now that really is magic!