Book: Pyrometrie

Overview
Johann Heinrich Lambert's Pyrometrie, published posthumously in 1779, presents a systematic attempt to bring measurement and mathematical clarity to the study of high temperatures and combustion. The text confronts the practical and conceptual difficulties of comparing degrees of heat that lie far beyond the reach of ordinary mercury thermometers. Lambert frames pyrometry as a science of reliable comparison, seeking reproducible procedures for registering and communicating intense heat as encountered in furnaces, forging, and chemical reactions.
Lambert treats heat not as a vague quality but as an observable cause of definite effects: change of volume, fusion and solidification, color and intensity of incandescence, and mechanical work. He insists that measurement must rest on stable, reproducible phenomena and builds his discussion around the selection, calibration, and use of such phenomena as "fixed points" or reference states against which other temperatures can be gauged.

Methods and Instruments
A central theme is instrumental design adapted to extreme conditions. Lambert evaluates several practical approaches to recording high temperature: the dilatation of solids and liquids, the progressive fusion of metals and alloys with known melting ranges, and optical methods based on the visible intensity and color of glowing bodies. He gives careful attention to material choice, proposing refractory substances that resist chemical alteration at high heat and emphasizing the need to limit contact between thermometer and hot medium to avoid contamination.
Optical pyrometry receives particular attention as a method that avoids physical intrusion into the hottest regions. Lambert explores how the brightness and tint of a heated surface correlate with its temperature when observed through an aperture or compared with a standard filament. He recognizes the complications introduced by differing emissivities and surface conditions and suggests ways to control or correct for them through comparative observation and calibration against metallurgical fixed points.

Theoretical Contributions
Lambert blends experimental practice with clear mathematical reasoning. He analyzes how measurable quantities, linear and volumetric expansion, latent changes at fusion, and luminous intensity, relate to an underlying scale of temperature. While acknowledging limits of contemporary theory about the nature of heat, he works to separate the notion of temperature as a comparative magnitude from the total quantity of heat present, clarifying the operational definition that allows consistent measurement.
Heat radiation and the propagation of warmth receive geometric and analytic treatment. Lambert applies principles of optics to the study of radiant heat, anticipating later ideas that link emission, absorption, and apparent brightness. His insistence on quantitative relationships and on correcting for geometric and material factors helps convert qualitative observation into a practicable metric enterprise.

Experimental Results and Fixed Points
Extensive experimental notes populate the text: careful determinations of melting points for various metals and alloys, records of expansion coefficients for candidate dilating bodies, and comparative trials of optical methods. Lambert compiles tables of reproducible reference events that can serve as anchors for an extended temperature scale. He emphasizes reproducibility over absolute interpretation, recommending well-documented procedures by which an observer can reproduce a calibration in another laboratory.
By proposing suites of fixed points that cover progressively higher ranges, Lambert provides a pragmatic road to extend temperature measurement well beyond existing thermometers. His experimental ethos stresses repeatability, clear description of apparatus, and the need to control chemical interactions that would otherwise corrupt readings.

Impact and Legacy
Pyrometrie helped to transform the study of high temperature phenomena from artisanal rules of thumb into a disciplined empirical science. Lambert's combination of meticulous experiment, mathematical clarity, and concern for practical instrument design influenced subsequent developments in thermometry, metallurgy, and the emerging field of thermal physics. His emphasis on fixed points, comparative methods, and correction for material and geometric effects anticipated later standardized temperature scales and the sophisticated pyrometric techniques used in industrial and scientific furnaces.
The treatise stands as a landmark in the 18th-century effort to render nature quantitatively accessible at extremes of condition, marrying hands-on experimental skill with the intellectual rigor that became a hallmark of modern physical measurement.