THERMOMETRY. 220 is 273.04, and so /° C. is (« X 273.04) " absolute. It is shown in Thermodynamics (q.v.) that there is a method of defining temperature which is quite independent of the thermometric substance, and in which the only arbitraiy things are the choice of a number for the ilitl'erence between the temperatures of any two thermal states. This ' scale is called 'Thomson's absohite scale,' be- cause it was proposed by William Thomson (now Lord Kelvin). If it is agreed to have the temperature of freezing and boiling water 100° apart, it is found that the numbers on tlic Thomson absohite scale agree most remarkably with those given on the absolute gas scale as defined above. History. The invention of the thermometer must be attributed to Galileo, who in about the year 1593 made an open-air thermoscope, con- sisting of a bulb with a long tube attached, which was provided with a scale and dipped below the surface of a liquid — water or wine: some of the air was expelled from the bulb, and so the liquid rose in the tube. This thermometer was used by Galileo for various purposes, such as studying freezing mixtures and recording at- mospheric temperatures. It was later used (1611) by Sanctorius in the diagnosis of fevers. The vi'ord 'thermoscope' was used by Bianeoni in 1617, and 'thermometer' in 1624 by Leurechon. The first scaled thermometers were those of Ferdinand II., Grand Duke of Tuscany (1641); these contained alcohol. In 1661 Fabri made a scale, using as the 'fixed' temperatures those of snow and of midsummer heat. Robert Hooke in 1664 proposed the freezing point of water as one of the fixed temperatures; and in 1694 Renal- dini proposed this as one, and the boiling point of water as the other. In 1709 Fahrenheit in- troduced his alcohol thermometers, and 1714 his mercury ones. About 1731 Rfeumur devised his scale, which until recently was in extensive use on the Continent of Europe. In 1742 Celsius proposed a Centigrade scale, with the tempera- tures of melting ice 100° and boiling water 0°. Christin, working independently of Celsius, pro- posed a Centigrade scale in 1743 which is the Celsius scale inverted, and is the one used now. A complete history of the ordinary thermometer is given in Bolton. Evohit'wn of the Thermometer (Easton, Pa.. 1900) ; in Abbe, Meteorolopical Ap- paratus and Methods (Washington, 18S7) ; and Gerland and Traumfiller. Geschichte der physika- lischen Experimentierkunst (Leipzig, 1899). See Thermometer. THERMOPHONE. See Thermometer. THERMOPILE. See Thermo-Electricitt. THERMOP'YL^ (Lat., from Gk. Sepixoir^Aat, from Sfpiidt, ttiennos; hot 4- 7ri)Xa, pi/Ia, gate) . A pass famous in ancient Greek history, lead- ing from Thessaly into Locris, named from the presence of .several hot springs. It is situated south of the present course of the river Sperchius, between Mount CEta and the Maliac Gulf. In ancient times it was only a narrow track, perhaps fifty feet wide, but the alluvial deposits have altered the coast line so that there is now a broad swampy plain from a mile and a half to three miles broad. Ther- mopylae has won an eternal celebrity as the scene oi the heroic death of Leonidas I. (q.v. ) and his three hundred Spartans in their attempt to stem THERMOTROPISM. the tide of Persian invasion (B.C. 480). But he was betrayed by Ephialtes, a Thessalian, into the hands of the Persians, who advanced by a path over the mountains, and fell upon his rear. In B.C. 279 a large army of Greeks held Brennua and his Gauls at bay until they also found a path over the mountains. The Greeks, however, escaped on their fleet. Again, in 3.c.'l91, Anti- oclius endeavored to check the Romans at this point, but Cato stormed the fortress which com- manded the path, and, aided by a frontal attack of the main force, routed the Syrian army. THERMOSCOPE (from Gk. Wp^i,,, therme, heat + CKoTTtlv, shopcin, to look ) . A device to indicate relative temperatures by making use of the property possessed by substances of changing their state or volume under the influence of heat. Thus, knowing the fusing-point of a solid, we are enabled to determine whether a certain tempera- ture is above or below the temperature at which it melts, while with a liquid we have two such points, namely, the temperature of vaporization and that of solidification. Such substances form what are termed discontinuous intrinsic thermo- scopes, and by virtue of their intrinsic properties enable us to ascertain relative temperatures. For example, butter would serve as such a ther- moscope, since by its melting it would indicate that the temperature of the room was above its point of fusing. Likewise we .soon use other sub- stances, including alloys, for this purpose, and ex- tend our observations over a considerable range. If the substance used as a thermoscope possesses the property of eontinuousl.v varying in its con- dition with an increa.se or diminution of tempera- ture, so as to afford a continuous indication over a given range of temperature, as is the case with mercury, then we have what is known as a con- tinuous thermoscope. If this is so arranged that it can be graduated according to some previously arranged scale, it then forms a thermometer. The camphor thermoscope is a sealed tube of glass containing a solution of camphor in alcohol, which indicates changes in temperature by its appearance. At high temperature the liquid is clear, as the camphor is entirely dissolved, but with low temperature it crystallizes and appears as a white fleecy mass. This instrument does not, as is commonly supposed, indicate changes either in pressure or electrical condition of at- mosphere. THERMOTROPISM (from Uk. eip^, therme, heat + t/soit^, trope, a turning, from Tp4ireiv, trepein, to turn). Primarily, the sen- sitiveness of plant organs to the direction of radiant heat which causes them to bend (posi- tively) toward or (negatively) away from its source. The same organ may be both positivel.v and negatively thermotr'opic under different conditions. It is often difficult to sepa- rate this phenomenon from that of heliotropism (q.v.), with which it is parallel, but there are several well-estaldished cases of thermotropism of roots and shoots of higher plants. If shoots of germinating maize or mustard seeds be placed in the dark and at the right distance from a suitable heat-radiating surface, they will bend according to the intensity of the heat either to- ward or away from the radiator. The term has also been used incorrectly to denote movements due to a change in temperature, such as those of
