Instruments and Facts

As the use of instruments characteristic of experimental physics spread to chemistry, new sorts of facts seized the chemist's attention. In 1750, Pott could say of fire as a chemical substance: "Although in its subtlety it cannot be investigated by number, measure, or weight, yet chemistry discovers a goodly number of its attributes."^[49] The impact of experimental physics changed matters. The means of production of chemical facts in themselves remained much the same—distillation, vaporization, and precipitation, according to the tradi-

[46] Pott, Chymische Untersuchungen, 1 , 60.

[47] See John Robison, "Preface," in J. Black, Lectures on the elements of chemistry , 2 vols. (Edinburgh: Mundell and Son, 1803), 1 , lxiv, which uses the term "pneumatic science."

[48] Henry Guerlac, "The continental influence of Stephen Hales," Archives internationales d'histoire des sciences, 15 (1951), 393–404, also in Essays, 275–84; and Lavoisier—the crucial years (Ithaca: Cornell University Press, 1961), 25–35. The methodological parallels between English pneumatics and Lavoisier have been stressed by J.R.R. Christie, "Ether and the science of chemistry," in G.N. Cantor and M.J.S. Hodge, eds., Conceptions of ethers. Studies in the history of ether theories, 1740–1900 (Cambridge: Cambridge University Press, 1981), 104.

[49] Pott, Chymische Untersuchungen, 1 , 61.

tional practices of assayers, pharmacists, and others in chemical trades—but, thanks to the influence of physics, facts yielded up by the balance assumed greater importance.

The major innovation at midcentury was not high accuracy in measurements, but rather numerical measurement per se. Exactness was not essential to the formulation of the theory of definite proportions. Proust derived his ideas about the chemical significance of proportions from his work in ordinary practical metallurgy, the inaccuracy of which left plenty of room for the debates between himself and Berthollet over the nature of chemical combination.^[50] The arguments central to Lavoisier's classical investigations on the supposed conversion of water to earth did not depend on great accuracy;^[51] they did, however, rest on a numerical base. Nor did his studies of fermentation indicate the importance of exact measurement in the concrete study of chemical processes. To be sure, Lavoisier gave the law of the conservation of mass in mathematical form in order to demonstrate its exactness, but he never came close to exactness in actual experiments.^[52] None was needed. The balance merely gave a gravimetric criterion for identifying and describing a unique chemical substance.^[53]

Lavoisier thus relied on a rhetoric of numbers. The complication of chemical reality, which could not be idealized, might have compromised the rhetoric. But Lavoisier and others explained away

[50] Cf. Seymour H. Mauskopf, "The Anales del Real Laboratorio de Quimica de Segovia of J.L. Proust," paper read at the XVIIth International Congress of History of Science (Berkeley, California, 1985); Satish C. Kapoor, "Berthollet, Proust, and proportions," Chymia, 10 (1965), 84–5.

[51] The balance lavoisier used was more exact than necessary: its precision was one-tenth of a grain (0.06 gram), but the "earth" in the pelican weighed 3 grains (0.15 gram) more than the loss of the pelican. See Partington, A history of chemistry, 3 , 377–80; Maurice Daumas, Scientific instruments of the seventeenth and eighteenth centuries and their makers (London: Batsford, 1972), 221–7.

[52] F.L. Holmes, Lavoisier and the chemistry of life (Madison: University of Wisconsin Press), 269, 281–3, 394–7, emphasizes that Lavoisier never tried to verify the law of conservation of mass, but that he assumed it. The significance of that law for a changing use of numbers in chemistry is therefore minimal.

[53] J.B. Gough, "Lavoisier and the fulfillment of the Stahlian revolution," Osiris, 4 (1988), 15–32; John G. McEvoy, "Continuity and discontinuity in the chemical revolution," ibid., 195–213.

large numerical discrepancies by invoking unknown chemical processes. In the water conversion experiment, for example, the numerical shortfall was blamed on a chemical reaction between the glass and the water.^[54] Still, the precision balance and the law of the conservation of matter conferred upon numbers a rhetorical value similar to what they enjoyed in physics and other fields during the late 18th century. Lavoisier made good use of the eloquence of the balance when arguing for the new chemistry.

Imponderables posed special technical problems. Lavoisier and Joseph Black wished to subject imponderables to quantitative study, but the usual array of chemical instruments offered no help. Other experimental devices were called for, such as the thermometer, which had not been an instrument for the chemist, and above all the calorimeter, recently constructed. These new instruments, introduced into chemistry from physics, became central to the study of chemically important substances. Deliberately constructed to yield quantitative results, they contributed to the introduction of numbers into chemistry. The study of heat stood at the intersection between physics and chemistry. Lavoisier and his physicist colleague Laplace met the problem of measuring the amount of heat participating in a chemical reaction by inventing the ice calorimeter.^[55] Bergman, both physicist and chemist, found a way to measure the relative phlogiston content of two metals. He knew that a metal lost its phlogiston in acid solution but could regain it when another metal was added to the solution. He therefore dissolved a certain weight of one metal in acid and then weighed the amount of a second metal necessary to precipitate entirely the first from solution. In his chemistry, the amounts of phlogiston in the two metals were proportional to the weights so determined.^[56]

[54] Stock, Development of the chemical balance , 2. Scheele's qualitative proof of the impossibility of transmutation was considered as valid as Lavoisier's quantitative demonstration. Cf. Partington, A history of chemistry, 3 , 380.

[55] Guerlac, "Chemistry as a branch of physics."

[56] Hugo Olsson, Kemiens historia i Sverige intill 1800 (Uppsala: Almqvist and Wiksell, 1971), 227f.