Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/146

134 P Y R P Y R this was not the case unless the coil was carefully protected by a platinum sheath. (8) Thermo-electric Methods. The measurement of 'high temper- atures by means of a thermo-electric junction has been attempted many times. A platinum-iron element was employed by Rosetti to measure the temperature of flames. 1 E. Becquerel- used a platinum- palladium element. The best-known results on the variation of the electro-motive force with temperature are those of Tait, 3 in the Edin. Phil. Traits. , xxvii. ; but full details of the measurement of temperature in his experiments are not given. It would appear, however, from Regnault's observations, 4 and from the well-known effect of slight differences in the physical state or composition of the metals used, that it is in every case necessary for the observer with a thermo-j unction to conduct his own comparison with an air-thermometer or other standard method. 8. The application of the variation in the wave-length of sound to the measurement of the density of air and con- sequent determination of the temperature has been sug- gested by Cagnard de Latour, Damon-Ferrand, Mayer, and Chautard. The method is liable to difficulties which need not be detailed here, but which are obviously sufficient to cause the experiments to be regarded rather as scientific curiosities than as pyrometric measurements. 9. Hitherto we have confined our attention to the ques- tion whether any instrument described is capable of giving trustworthy indications of the temperature of the instru- ment itself ; in order to be satisfied as to whether they fulfil their object, we have still to consider whether they can be easily made to take up the temperature of the body or enclosure under investigation. This is a very difficult question, and it seems doubtful whether with such an instrument as Siemens's electrical pyrometer, of which the coil is contained in a massive sheath of iron connected to about 6 feet of stout iron tube, thermal equilibrium between the coil and the enclosure is possible. We have not space to discuss the matter, but it seems not unlikely that the differences which still exist between the results of different observers may be due to the method of exposure of the pyrometer. In connexion with this the researches of Regnault 5 with reference to the determination of the boiling-point of mercury and sulphur are very important. He observed that his thermometers, when exposed directly to the steam, indicated too high a temperature, and that it was therefore necessary for the socket enclosing the ther- mometer to dip into the liquid to such an extent that the surface of the liquid was above the level of the top of the thermometer bulb. Whether or not this may account for some of the differences between the results obtained for the boiling-point of zinc by Deville and Troost and by E. Becquerel and Violle it is difficult to say. The following table (III.) will show the divergence among the best of the high-temperature measurements. Table ITT. Determinations of the Soiling-Point of Zinc. Pressure. Temp. Method. Observer. Reference. 759-54 mm. 1039 Iodine vapour pyro- Deville and Troost, 3, (2) meter 1859 761-2 1040 932* Platinum reservoir E. Becquerel 1863 3,' (3) air-thermometer 891 Porcelain reservoir air-thermometer 718-9 1035 Porcelain reservoir Weinhold, 1873 .... 3, (3) air-thermometer 929'-954 Air-thermometer . . Deville and Troost, 3, (1) 1880 916 -925" Hydrogen - thermo- it tj meter 760 930' Porcelain reservoir Violle, 1882 3, (3) air-thermometer 10. Perhaps the most important modern attempts at the development of pyrometry are those connected with the identification of the law connecting the temperature of a body with the amount and nature of the energy which 1 Ann. de Chim., 1878. 2 Ann. de Chim., Ixviii. p. 49. 4 Mem. de I'Inst., xxi. p. 241. 3 See ELECTRICITY. 5 Mem. de I'Inst., xxvi. p. 513. it radiates. On such attempts depends the possibility of measuring the temperature of a hot body by means of the light it emits. This is evidently a most desirable object, since, if that were possible, one of the great difficulties of pyrometry the bringing of the measuring instrument to the temperature of the body under investigation would immediately disappear. At present, however, there is no general agreement among scientific men as to the form the relation takes. We cannot here do more than refer to the "Report on Spectrum Analysis," in the British Asso- ciation's Reports for 1881 and 1884, for references to the literature of the subject. See RADIATION. (w. N. s.) PYROTECHNY is the art of producing pleasing scenic effects by means of fire. It is not held to include the manufacture of inflammable and explosive substances for other purposes. The use of fireworks for purposes of display is not a modern invention, for it appears to have existed in China in very ancient times ; but the secret of constructing them remained unknown in Europe till about the 13th century, when the knowledge of GUNPOWDER (q.v.) crept in from the East. In modern times the art has been gradually improved by the work of specialists, who have had the advantage of being guided by scientific knowledge. The value of such knowledge to the pyro- technist is extremely great ; for he must be governed by the principles of chemistry in the selection of his materials, and his various contrivances for turning them to the best account are subject to the laws of mechanics. As in all such cases, however, science is useless without the aid of practical experience and acquired manual dexterity. Many substances have a strong tendency to combine with oxygen, and will do so, in certain circumstances, so energetically as to render the products of the combination (which may be solid matter or gas) intensely hot and luminous. This is the general cause of the phenomenon known as fire. Its special character depends chiefly on the nature of the substances burned and on the manner in which the oxygen is supplied to them. As is well known, our atmosphere contains oxygen gas diluted with about four times its volume of nitrogen ; and it is this oxygen which supports the combustion of our coal and candles. But it is not often that the pyrotechnist depends wholly upon atmospheric oxygen for his purposes ; for the phenomena of combustion in it are too familiar, and too little capable of variation, to strike with wonder. Two cases, however, where he does so may be instanced, viz., the burning of magnesium powder and of lycopodium, both of which are used for the imitation of lightning in theatres. Nor does the pyrotechnist resort much to the use of pure oxygen, although very brilliant effects may be produced by burning various substances in glass jars filled with the gas. Indeed, the art could never have existed in anything like its present form had not certain solid substances become known which, containing oxygen in combination with other elements, are capable of being made to evolve large volumes of it at the moment it is required. The best examples of these solid oxidizing agents are nitrate of potash (nitre or saltpetre) and chlor- ate of potash ; and these are of the first importance in the manufacture of fireworks. If a portion of one of these salts be thoroughly powdered and mixed with the correct quantity of some suitable combustible body, also reduced to powder, the resulting mixture is capable of burning with more or less energy without any aid from atmo- spheric oxygen, since each small piece of fuel is in close juxtaposition to an available and sufficient store of the gas. All that is required is that the liberation of the oxygen from the solid particles which contain it shall be started by the application of heat from without, and the action then goes on unaided. This, then, is the funda-