Inventing Temperature Read online

  17977

  Welding heat of iron, greatest

  95

  13427

  Welding heat of iron, least

  90

  12777

  Fine gold melts

  32

  5237

  Fine silver melts

  28

  4717

  Swedish copper melts

  27

  4587

  Brass melts

  21

  3807

  Red-heat fully visible in daylight

  1077

  Red-heat fully visible in the dark

  −1

  947

  Mercury boils

  −3.673

  600

  Source: Wedgwood 1784, 370.

  Source: Adapted from the second series of data given in Regnault 1847, 188.

  there was sufficient uniformity in the clays found in various places at equal depths. If problems arose, he thought that all the pyrometer pieces ever needed could be made with clay from one particular bed in his own possession in Cornwall: "[T]he author offers to this illustrious Society [Royal Society of London], and will think himself honoured by their acceptance of, a sufficient space in a bed of this clay to supply the world with thermometer-pieces for numerous ages." But to his great chagrin, Wedgwood (1786, 398-400) later found out that different samples of clay even from that same area of Cornwall differed from each other in their thermometric behavior in uncontrollable ways. In short, even Wedgwood himself had trouble reproducing the "standard clay" pieces that he had initially used. The shrinkage behavior became more controllable when he employed a slightly artificial preparation, a mixture of alum and natural clay, but in the end Wedgwood had to resort to the use of several fixed points to ensure sufficient comparability (0°W at red heat as before, 27°W at the melting point of silver, 90°W at the welding heat of iron, and 160°W at the greatest possible heat of a good air-furnace).36

  The other major difficulty, which Wedgwood never addressed in print, concerned the connection with ordinary scales of temperature. Many of Wedgwood's critics did not believe that he had worked out the Wedgwood-Fahrenheit conversion correctly, in three different ways. These points will be discussed in more detail in the next section: 1.

  His estimate of the temperature of red heat (the beginning and zero point of his scale) was too high.

  2.

  His estimate of the number of Fahrenheit degrees corresponding to one degree of his scale was also too high.

  3.

  There was no positive reason to believe that the contraction of clay was linear with temperature.

  36. For a description of the alum mixture, see Wedgwood 1786, 401-403; for the fixed points, p. 404.

  end p.126

  Figure 3.5. Late nineteenth-century comparison of Wedgwood and centigrade degrees. The data represented in the figure are as reported in Le Chatelier and Boudouard 1901, 164.

  These points are supported by later appraisals, summarized by the physical chemist Henri Louis Le Chatelier (1850-1936) in the late nineteenth century, shown graphically in figure 3.5.

  If all the points of criticism mentioned earlier are indeed correct, the most charitable light in which we can view Wedgwood pyrometry is that it gave some rough indication of high temperatures, but without conceptual or quantitative accuracy. Some later commentators have used this apparent failure of pyrometry as evidence that Wedgwood was scientifically unsophisticated. But the first set of problems, namely those concerning the lack of uniformity in the behavior of clay, cannot be held against Wedgwood, since he recognized them clearly and devised very reasonable methods for dealing with them. The second set of problems is a different matter. When Neil McKendrick (1973, 280, *310) disparages Wedgwood's pyrometry for "its obvious scientific shortcomings" and "its lack of scientific sophistication and lack of command of theory," the chief fault that he finds is Wedgwood's "failure to calibrate its scale with that of Fahrenheit." McKendrick surely could not be missing the fact that Wedgwood did calibrate his thermometric scale with the Fahrenheit scale, so what he means must be that Wedgwood did it incorrectly.

  But how can we be so sure that Wedgwood was wrong? And, more pertinently, how can we be sure at all that any of the proposed alternatives to Wedgwood pyrometry were any better? It is quite true that Wedgwood had not produced any direct empirical justification for his assumption that the expansion and contraction of his clay and silver pieces depended only on temperature and linearly on temperature. But these assumptions could not be tested without an independent method of measuring the temperatures involved, and there were none available to

  end p.127

  Wedgwood. He was striking out into virgin territory, and no previous authority existed to confirm, or contradict, his reports. On what grounds did his opponents declare his numbers false? That is the great epistemic puzzle about the downfall of the Wedgwood pyrometer. We need to examine with some care the process by which physicists, chemists, and ceramic technologists came to rule against Wedgwood, and the character of the alternative standards with which they replaced Wedgwood's pyrometer. (The doubts were raised strongly only after Wedgwood's death in 1795, so we can only speculate on how the master artist himself would have responded.)

  Ganging Up on Wedgwood

  In discussing the work of Wedgwood's critics, I will depart slightly from the chronological order and organize the material in terms of the alternative pyrometric methods they proposed and developed. I will discuss the alternatives one by one, and then assess their collective effect. To anticipate the conclusion: I will show that each of the temperature standards favored by Wedgwood's critics was as poorly established as Wedgwood's own. Their main strength was in their agreement with each other. What exactly such a convergence of standards was capable of underwriting will be discussed fully in the analysis part.

  The Expansion of Platinum

  This alternative to Wedgwood pyrometry was conceptually conservative but materially innovative. It hinged on a new material, platinum. Although platinum was known to Europeans since the mid-eighteenth century, it was only at the beginning of the nineteenth century that William Hyde Wollaston (1766-1828), English physician and master of "small-scale chemistry," managed to come up with a secret process to render it malleable so that it could be molded into useful shapes and drawn into fine wires.37 As platinum was found to withstand higher degrees of heat than any previously known metals, it was naturally tempting to enlist it in the service of pyrometry. The simplest scheme was to use the thermal expansion of platinum, in the familiar manner in which pre-Wedgwood pyrometry had used the expansion of various metals.

  The clear pioneer in platinum pyrometry was Louis-Bernard Guyton de Morveau (1737-1816), who had probably one of the most interesting scientific and political careers through the French Revolution and Empire. A prominent lawyer in Dijon whose reforming zeal had drawn Voltaire's praise, Guyton became gradually swept up in revolutions both in chemistry and politics. Having collaborated with Lavoisier on the new chemical nomenclature, Guyton threw himself into the political revolution that would later claim the life of his esteemed colleague. He moved to Paris

  37. This process gave Wollaston a comfortable income for the rest of his life. Much further useful information can be found in D. C. Goodman's entry on Wollaston in the Dictionary of Scientific Biography, 14:486-494.

  end p.128

  as a member of the National Assembly and then the Convention, dropped the aristocratic-sounding "de Morveau" from his name, voted for the execution of the king, and served as the first president of the Committee of Public Safety until he was removed as a moderate with the coming of the Reign of Terror. He did return to the committee briefly after the fall of Robespierre, but soon retired from politics and concentrated on his role as a senior statesman of science. Guyton was one of the first members of the new French Institute at its founding, and the president of its First Class (mathematical and physical sciences) in 1807. At the École Polytechn
ique he was a professor for nearly twenty years and director twice.38

  The chemist formerly known as De Morveau started working with Wedgwood pyrometers in his research on the properties of carbon published in 1798 and 1799, which reported that charcoal was an effective insulator of extreme heat and that diamond burned at 2765°F according to Wedgwood's pyrometer and conversion table. He announced at that time that he was engaged in research toward improving the pyrometer.39 In 1803 he presented his platinum pyrometer to the French Institute and promised to present further research on its relation to the Wedgwood pyrometer. Guyton's comparison of the platinum pyrometer with the Wedgwood pyrometer led him to propose a serious recalibration of the Wedgwood scale against the Fahrenheit scale, bringing 0°W down to 517°F (from Wedgwood's 1077°F), and estimating each Wedgwood degree as 62.5°F (not 130°F as Wedgwood had thought).40 The overall effect was to bring Wedgwood's temperature estimates down considerably; for instance, the melting point of cast iron was adjusted from 17,327°F down to 8696°F (see the first two columns of data in table 3.2). Guyton does not specify explicitly how exactly his recalibration was done, but judging from the data it seems as though he fixed the clay scale to agree with the platinum scale at the melting points of gold and silver. Guyton was well aware that there was no guarantee that the expansion of platinum at high temperatures was linear with temperature, and his reasons for trusting the platinum pyrometer over the Wedgwood pyrometer rested on the agreement of the former with a few other types of pyrometers, as we will see shortly.

  No significant progress on platinum pyrometry seems to have been made after Guyton's work until John Frederic Daniell (1790-1845) made an independent invention of the platinum pyrometer in 1821, nearly two decades after Guyton. At that point Daniell was best known as a meteorologist, although later he would achieve more lasting fame in electrochemistry largely with the help of the "constant battery" that he invented himself. He spent the last fifteen years of his life as the first professor of chemistry at the newly established King's College in London, widely respected for his "lofty moral and religious character" as well as his successes

  38. For these and many further details, see W. A. Smeaton's entry on Guyton de Morveau in the Dictionary of Scientific Biography, 5:600-604.

  39. See also the note Guyton attached (pp. 171-172) in communicating Scherer 1799 to the Annales de chimie.

  40. See Guyton 1811b, 90-91, and also table 3.

  end p.129

  Table 3.2. A comparative table of data produced by various pyrometric methods, published up to the first half of the nineteenth century

  Clay °Wa

  Conversion into °F

  Mercury

  Metal

  Ice

  Water

  Air

  Cooling

  Current values

  Melting point of tin

  481 (N)

  441 (Da)

  383 (C/D)[G]

  449 [L]

  415 (B)b

  442 (G)

  410 (Pa)

  455 (Pc)

  Melting point of bismuth

  537 (N)

  462 (Da)

  662 (C/D)[G]

  521 [L]

  494 (B)b

  476 (G)

  493 (Pa)

  518 (Pc)

  Melting point of lead

  631 (N)

  609 (Da)

  617 (C/D)[G]

  621 [L]

  595 (B)b

  612 (G)

  500 (Pa)

  630 (Pc)

  Melting point of zinc

  3 (G)

  705 (G)

  699 (B)b

  932 (C/D)[G]

  787 [L]

  680 (G)

  680(Pa)

  648 (Da)

  793(Pc)

  773 (Di)

  Red heat visible in the dark

  947 (W)

  743 (N)

  977 (Dr)

  Melting point of antimony

  7 (G)

  955 (G)

  809 (B)b

  847 (C/D)[G]

  942 (N)

  1167 [L]

  810 (G)

  810 (Pa)

  Red heat visible in daylight

  0, by definition

  1077 (W)

  1050 (B)b

  1272 (C/D)[G]

  977 (Pb)

  1036 (N)

  517 (G)

  1200 (Pr)

  980 (Da)

  Melting point of brass

  21 (W)

  3807 (W)

  1869 (Da)

  1706-1913 [R]

  21 (G)

  1836 (G)

  Melting point of silver

  28 (W)

  4717(W)

  1000 (B)b

  1000 (Pa)

  1763 [C]

  22 (G)

  1893 (G)

  1893 (G)

  1832 (Pb)

  1761 [R]

  2233 (Da)

  1830 (Pr)

  1763 [L]

  1873 (Db)

  1682 (Di)

  Melting point of copper

  27 (W)

  4587 (W)

  1450 (B)b

  2295 (C/D)[G]

  1984 [C]

  27 (G)

  2205 (G)

  2313 (G)

  1981 [R]

  27 (Pa)

  2548 (Da)

  1984 [L]

  1996 (Db)

  Melting point of gold

  32 (W)

  5237 (W)

  1301(B)b

  2192 (Pb)

  1948 [C]

  32 (G)

  2518 (G)

  2518 (G)

  2282 (Pc)

  1945 [R]

  32 (Pa)

  2590 (Da)

  1948 [L]

  2016 (Db)

  1815 (Di)

  Welding heat of iron, least

  90 (W)

  12777 (W)

  1922 [R]

  95 (G)

  6504 (G)

  Welding heat of iron, greatest

  95 (W)

  13427 (W)

  2192 [R]

  100 (G)

  6821 (G)

  Red hot iron

  88 (C/D)[G]

  12485 (C/D)[G]

  2732 (C/D)[G]

  White hot iron

  100 (C/D)[G]

  14055 (C/D)[G]

  3283 (C/D)[G]

  Melting point of cast iron

  130 (W)

  17977 (W)

  1601 (B)

  3164 (C/D)[G]c

  1922-2192 (Pb)

  2100-2190 [R]

  130 (G)

  8696 (G)

  3479 (Da)

  2786 (Db)

  melting point of soft iron

  174 (C/D)[G]

  23665 (C/D)[G]

  3988 (C/D)[G]

  3902 (C/D)[G]

  175 (G)d

  11455 (G)

  2700-2900 (Pb)

  Melting point of steel

  160 [R]

  ~2370-~2550 (Pb)

  154 [R]

  Greatest heat, air furnace

  160 (W)

  21877 (W)

  170 (G)

  Melting point of platinum

  unknown (G)

  over 3280 (Db)