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Before the genesis of the science of pyrometry and the construction of laboratory electric furnaces capable of accurate temperature control, observations on natural silicates and artificial silicates, such as glasses, were confined to determinations at ordinary temperatures of such physical properties as density, coefficient of expansion, hardness, thermal conductivity, specific heat, refractive index, dielectric constant, optical rotary power, and coeffi cient of absorption. A knowledge of these properties, while indispensable in certain cases, cannot serve the purpse of elucidating the physical properties of the silicates at elevated temperatures. H. Le Chatelier (Note 1) has recently published in book form an excellent treatise on the general properties of silica and the silicates, which also contains brief reference to the earlier high-temperature investigations.

The scientific investigation of the silicates at high temperatures may be considered to have had its origin in 1880, when Dr. Carl Barus was appointed physicist of the United States Geological Survey for the purpose of determining the formation temperatures of the rocks which form the earth's crust (2). Barus devoted himself in part to the development and application of the thermocouple to high-temperature measurement at a time when Le Chatelier in France was attacking the problem independently. This work received a new impetus in 1900, when Dr. Arthur L. Day took up the work and introduced the high-temperature scale and methods of the Reichsanstalt into the United States Geological Survey. Soon after this date these investigations were taken over by the Carnegie Institute of Washington, which resulted later in the founding of the well-known Geophysical Laboratory.

peratures in the Kilns of the Ceramic Industries." In 1888 Callendar (4) described his improved form of the Siemens resistance thermometer, while Joly (5) in 1891 invented the micropyrometer which bears his name. In 1892 Le Chatelier (6), acting upon a suggestion originally made by Becquerel (7), devised the optical pyrometer which bears his name, which, according to recent experiments by Waidner and Burgess (8), is as accurate in its measure. ment of temperature as other more recent types, such as the Wanner (9) (1901) and the Féry (10) absorption (1904) optical pyrometers. About the same time Holborn and Kurlbaum (11) in Germany and Morse (12) in the United States brought out an optical pyrometer using a new photometric method. The former of these, the HolbornKurlbaum, is generally considered to be the most precise instrument on the market, and is, according to Waidner and Burgess (13), capable of a precision of 2° at 1500° C. In 1902 Féry (14) devised a pyrometer utilising the energy of total radiation.

Joly (15) determined the melting-points of a number of minerals by means of his novel apparatus, temperatures being estimated by the linear expansion of the strip of heated platinum foil upon which the small test grains were placed. The melting point was estimated by noting the occurrences of deformation or incipient fusion, and hence should be classed as a softening temperature rather than a melting-point. In 1901 Cornelio Doeiter (16) began the publication of an extended series of experiments on the melting-points of minerals, using thermocouples for measuring temperature and electric furnaces modelled after those of the Reichsanstalt, as used by Holborn and Day. Doelter, as did Joly, used a purely subjective method for estimating the melting point, and recorded two temperatures the first approach of viscous melting and the point where the material appeared to have gone over into a thin liquid. It appears, then, that both Joly and Doelter have determined the temperature at which the various minerals investigated approached a more or less definite viscosity, rather than the true physical meltingpoint.

Since 1905, when Messrs. Day and Allen (17) published their important work on "The Isomorphism and Thermal Properties of the Feldspars," the contributions from the Geophysical Laboratory of Washington on the meltingpoints and the stability relations of the silicates at high temperatures have formed the greater part of authoritative high-temperature research on the properties of the silicates, most of these publications having appeared in the American Journal of Science.

In view of the comparatively short existence of accurate pyrometry and of accurately controlled laboratory furnaces, it is uot surprising that Sir Lowthian Bell in his admirable investigations on the manufacture of pig iron in the blastfurnace, which were published in 1884 (18), was seriously handicapped in so far as the determination of high temperatures was concerned; and also that Akermann (19), in his painstaking determination of the "total heat" of slags by the calorimetric method, was not able at that time (1886) to estimate the temperature of the slag melts which were investigated, but rather was forced to refer his measurements to a more or less indefinite "pouring temperature."

A Paper communicated to the Transactions of the Faraday Society. Published by permission of the Director, U.S. Bureau of Mines.