Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/141

Rh P Y R P Y R 129 most important variety of the mineral is a " cupreous iron pyrites," which for many years past has been wrought on an enormous scale in Spain and Portugal. This ore seems to be an intimate mixture of iron pyrites with a small quantity of copper pyrites, the proportion of metallic copper being generally less than 3 per cent. Notwithstanding the poorness of the ore, the copper is profitably extracted by wet processes. There is also present a small quantity of silver (20 to 35 dwt. per ton), with a trace of gold. The deposits of this cupreous pyrites are of enormous magni- tude, and occur at the junction of porphyritic rocks with clay-slate of Devonian age. The principal Spanish mines are those of Rio Tinto, Tharsis, and Calanas in the province of Huelva ; whilst the most important of the Portuguese mines is that of San Domingos in the province of Alemtejo. There is ample proof that some of these pyritic deposits were worked by the ancient Romans. The quantity of cupreous and other iron pyrites imported into Great Britain during the year 1883 principally from Spain and Portugal, but partly from Norway and elsewhere was 601,288 tons, of the declared value of 1,356,083. But this quantity had been exceeded in several previous years, notably in 1880 and 1877. The quantity of iron pyrites raised in the United Kingdom in 1883 amounted to 27,672 tons, of the value (at the mine) of 17,467. See IROX, vol. xiii. pp. 280, 288 ; COPPER, vol. vi. p. 347 ; MAR- CASITE, vol. xv. p. 532 ; and MINERALOGY, vol. xvi. pp. 390, 393. For details of the Rio Tinto pyrites, see A Treatise on Ore-Deposits, by J. Arthur Phillips, 1884. PYRMONT. See WALDECK. PYROMETER, an instrument for measuring high temperatures. As long ago as 1701, in a paper l published anonymously in the Philosophical Transactions, Newton gave the results of attempts to estimate the temperature of red-hot iron by noting the time it took to cool to an observed temperature, assuming what has since been called Newton's Law of Cooling. The numerical results are given in terms of the degrees of a linseed-oil thermometer constructed by Newton. Its zero was the temperature of melting ice and its second fixed point the normal temper- ature of the human body, denoted by 12. About the same time Guillaume Amonton in Paris made somewhat similar attempts to determine the temperature of the red-hot end of an iron bar, using for reference a rudimentary air-thermo- meter the first of its kind in which the variation of atmospheric pressure was allowed for. Since the middle of the last century the different methods and instruments suggested for measuring high temperatures have been very numerous, in fact the variation of almost every physical property of substances which alter with change of temper- ature has been utilized for this purpose. Measurements of the increase of pressure produced in a quantity of gas while its volume remains constant or of the increase of volume at constant pressure,' of the heat given out by a mass of metal in cooling to an observed temperature, of the expansion of a metal or graphite bar or of a mass of clay are those which have been most frequently employed ; but, besides these, the change in the electrical resistance of a wire, the saturation-pressure of the steam of various liquids, the pressure of gas dissociated from various solids, the electromotive force of a thermo-electric couple, the density of the vapour of a liquid, the change of shape of a compound spiral of different metals, have been used, even the alteration in the wave-length of a note of given pitch has been suggested as capable of being made use of for pyrometric purposes. For reasons which will be given below, the numerical results obtained by one or other of the numerous forms of the gas-thermometer have a more definitely intelligible value. The gas-thermometer 1 "Scala GraJuum Caloris,"in Phil. Trans., xxii. p. 824. method and the calorimetric method were both employed by Pouillet for the accurate measurement of high temper- atures before 1836. 2. The indications obtained by any of the numerous methods which have been suggested are, as a rule, expressed in terms of Centigrade or Fahrenheit degrees. This assign- ment of numbers presupposes not only a definition of tem- perature by which the size of the degree is determined but also a physical law which gives the relation between the measured interval of temperature and the standard degree. The various definitions of the standard degree that might be employed will be found in the article HEAT, sees. 12, 24, 25, 30, 31, 32 ; and in sec. 35 of the same article the definition of the absolute thermodynamic scale of temper- atures is given. In the same article (sec. 38) it is shown that the "absolute temperature" of a liquid in thermal equilibrium with its own vapour under a pressure p may be obtained from the formula ' p (l-ff)dp JpK I where p is the density of the vapour, per that of the liquid, K the latent heat per unit-mass of the vapour corresponding to the saturation -pressure p. The dynamical equivalent of heat is represented by J. We have therefore the com- plete theory of what may soon become a practical method of expressing temperatures in the thermodynamic scale. Sir W. Thomson, in the article mentioned (sees. 39-45), has described arrangements for measuring the pressure of the saturated vapours of various liquids which will give that measurement in a thoroughly satisfactory manner up to, at any rate, some 600 C. For the higher tempera- tures mercury is the liquid employed. There are, however, some experimental data still wanting before the formula quoted above can be applied to the numerical calculation of the temperature. These are (1) the density p of the saturated vapour corresponding to the series of pressures, and (2) the corresponding latent heat K of vaporization. These constants have not yet been actually observed. Instruments such as those figured in the article cited can, however, be employed with convenience and accuracy as continuous intrinsic thermoscopes, whose indications can supply a numerical measure of temperature after an empirical graduation. When used thus they possess the enormous advantage that the pressure of the saturated vapour at a definite temperature is perfectly definite, so that a single observation of the pressure is all that is necessary to determine the temperature, and the instru- ment can be easily arranged, so that this observation is practically a very simple one. The pressure of mercury vapour has already been determined by Regnault for temperatures up to 550. A thermoscopic method of pyrometry which is very similar to the above was sug- gested by Lamy. 2 He proposed to measure the pressure of carbonic -acid gas dissociated from calcium carbonate. There is experimental evidence to show that the pressure of the dissociated gas is definite at a definite tempera- ture. The recombination of the dissociated gas with the solid is, however, a slow process, and the method has been pronounced by Weinhold 3 to be practically unsatis- factory. 3. G as Pyrometry. Measurement of High Tempera- tures by the Expansion of Air and other Gases and Vapours. Temperatures may be expressed in the absolute thermo- dynamic scale by the method of the gas -thermometer, which is available for practical purposes even at very high temperatures. It has been shown 4 that the indications 2 Comptes Rendus, Ixix. p. 347. 3 " Pyrometrische Versuche," Pojg. Ann., cxlix. p. 186. 4 See HEAT, sees. 46-67. XX. 17