In 1946 George Orwell published his 11 golden rules to make a perfect cup of tea.
Needless to say, I am usually careless about them when I make my own "tea". Back in Madrid I had one electrical kitchen heater and three gas ones. I always used the electrical, a dry plate which got very, very hot for cooking. I normally let the whole thing boil as it pleased and in many occasions, liquid spurred out of the tea pot neck in splashes, clang to the side and slid down the pot on to the hot plate. The liquid accumulated there; so when I removed the tea from the heat to pour it into the cup (tea-pot effect happened here), the remaining liquid on the plate formed neat, perfect spheres. The spheres moved like crazy in tiny, random movements (like the motion of a plastic ball suspended at a short height from the ground by an air stream), and their sizes were diminishing until they vanished.
I always thought it was kind of cute but really did not put a lot of thought about that.
Today I learned that somebody actually did it _wrong, but did it.. I was in the lab while waiting for any student that might come to ask me questions about the experiment (office hours). I had just checked out this book from the library, brought out by Hemisphere Publishing Company (1981) and was looking over the last chapter: a nice review of the history of gas - liquid flow and heat transfer, by Professor Arthur Bergler. The guy got his PhD from the MIT in 1962.
Johann G. Leidenfrost (1715 - 1794) was a Prusian doctor who, despite his first travels and his work as a field doctor during the first Silesian War, was appointed as a medical professor at the University of Duisburg, a city today to the very west of Germany, north Dusseldorf and close to the Neatherlands border. His work De Acquae Communis Nonnulis Qualitatibus Tractatus, published in 1754 is regarded as the first serious attempt to approach the boiling phenomena.
I wish to draw attention on to this wonderful translation, by Carolyn Wares (1966), University of Oklahoma, I heard:
"On the Fixation of Water in Diverse Fire:
An iron spoon (...) is heated over glowing coals (...) To this glowing spoon (...) send (...) one drop of very pure distilled water (...). This water globule (...) delights in a very swift motion of turning (...). Moreover, this drop only evaporates very slowly (...). When at last exceedingly diminished so that it can hardly any more be seen, with an audible crack, which with the ears one easily hears, it finishes its existence (...) These observations suggest that water is changed into earth by a large fire, because always after the complete evaporation of the drop, some terrestrial matter remains in the heated vessel(...)".
(...) Which with ears one easily hears... . I love it! How much arid is scientific language now! Science must have been just born. It is noticeable, also, that the discovery beyond any doubt of water as a compound was not done until the decade of 1780s, and that gives a nice context for the "fixation" and the alchemical conclusion: water is changed into earth by a large fire. How nice!
It was suggested at the time, as well, for those opposing to the Fixation that dust swinging around the lab might enter the drop and that would be the "terrestrial matter" remaining in the vessel. However, this is hard to believe. Leidenfrost mentions that the drop is prepared from "very pure distilled water". The spoon is polish and free from rust and dirt. What a horrible lab it must have been to convey that appreciable amount of dust in only about 30 seconds (time for the drop to vanish).
Today, it is known that the "terrestrial matter" pertains to crud deposition: corrosion residual unindentified deposit.The Leidenfrost effect is explained by a layer of vapor that forms from rapid contact with a hot surface and that surrounds the liquid in a drop, retarding its boiling time. That steam actually corrodes the metal of the plate in that short time.
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