Free Absolute Zero and the Conquest of Cold by Tom Shachtman

mercury and recalibrated his scale. On his newer thermometers, each degree corresponded to one ten-thousandth the initial volume of the mercury, the same proportion as in Boyle's and Newton's thermometers. To achieve that ratio, he had to quadruple the values on his scale—which turned out well, because with the melting-ice point at 32° and the blood-heat point at 96°, he now had a scale on which the key numbers were still divisible by 4, but on which the range was greater, making it easy to work with. The ability to more accurately locate blood heat was quite important, because Fahrenheit knew of Boerhaave's interest in measuring body temperature and wanted his influential patron to adopt (and recommend) his devices.
    Perhaps to preserve his ability to exclusively manufacture these thermometers, Fahrenheit did not publish the calculations that had led to his scale. Knowles Middleton, the modern authority on the history of thermometers, suggests that all instrument makers concealed such matters, or obfuscated them, to prevent others from replicating their instruments without paying for them. Thus while Fahrenheit promised to provide Boerhaave with "accurate descriptions of all the thermometers which I make, and of the way in which I have ... attempted to rid them of their defects, and by what means I have succeeded in doing so," he actually withheld the secret of the volumetric measurements that had helped him arrive at the important numbers of 0, 32, and 96. An unintended consequence of this concealment was that for hundreds of years afterward, scientific historians wrote that Fahrenheit's scale was arbitrary.
    In 1724 Fahrenheit was elected as a foreign member of the Royal Society and went to London. Just before this trip, he did some significant though haphazard basic experiments to determine some fixed points for his thermometers. In a 1729 letter to Boerhaave, Fahrenheit first apologized for any delay in delivery of his thermometer, saying he was "now seeking a friend of the fair sex, which matter, as you will understand, will take up much of my time until it is resolved," then told of his pre-1724 research on the artificial production of cold.
    These experiments were like a chance visit to an unknown territory by someone unprepared to be a proper explorer but who grasped the opportunity of an accidental landing to climb a mountain, view the lay of the land, and record a few observations of unusual phenomena. Mixing aquafortis (concentrated nitric acid) and ice, Fahrenheit reduced the temperature of his mix to a point so low that the largest measuring tube he had was not long enough to measure it. Intrigued, he constructed a thermometer able to register as far down as 76 below his 0 and managed to lower the temperature of an aquafortis/ice mixture to 29 below.
    Next, using a series of beer glasses with freezing mixtures in them, he tried what would later become known as a cascade series: he cooled the first glass with his original aquafortis/ice mixture until the mix registered its lowest temperature; then he poured off the liquid in the mixture and used the solid that remained on the bottom as the starter for the second glass, to which he added more liquid, enabling him to reduce the temperature of the second glass to 32 below; continuing the technique, he reached down successively to 37 and then to 40 below. He boasted to Boerhaave that he could have gone still lower had he had purer chemicals. During this exploratory descent, Fahrenheit also managed to create crystals from some liquids. Not well enough schooled to make advanced scientific conclusions from his experiments, he "humbly" wrote to Boerhaave the prescient observation that "we know just as little of the first commencement of heat as we know of the extreme limit of heat."
    After his sole excursion into the territory of the cold, Fahrenheit returned to what he did best, the manufacture of measuring implements, and confided in his next letter to