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THERIDIID-ffi 346 THERMO CHEMISTBY and the greater number belong to the Eastern Hemisphere. THERIODONTIA, in palaeontology, an order of Reptilia, founded by Owen for the reception of a number of re- mains from deposits in South Africa of Triassic or Permian age. The denti- tion is of the carnivorous type, consisting of incisors, canines, and molars. THERIOMORPHA, in zoology, Owen's name for the tailless amphibians (frogs and toads), more generally called Anoura, or Batrachia Salientia. It is a synonym of Huxley's Batrachia, a name used by Owen to designate the class Amphibia. THERMS. In the Roman thermae, the baths were of secondary importance. They were a Roman adaptation of the Greek gymnasium, contained exedrse for the philosophers and rhetoricians to lec- ture in, porticoes for the idle, and libra- ries for the learned, and were adorned with marbles, fountains, and shaded walks and plantations. THERMIDOR, i. e., the "Hot Month," formed, in the calendar of the first French Republic, the 11th month, and lasted from July 19 to Aug. 18. The 9th Thermidor of the Republican year 2 (July 27, 1794) is historically memorable as the date of Robespierre's fall, and the termination of the Reign of Terror. The name Thermidorians was given to all those who took part in this fortunate coup d'etat, but more particularly to those who were desirous of restoring the monarchy. THERMO BAROMETER, an instru- ment for measuring altitudes by means of determining the boiling point of wa- ter. They consist essentially of a small metallic vessel for boiling water, fitted with very delicate thermometers, which are only graduated from 80 to 100°; so that each degree occupying a consid- erable space on the scale, the tenths, and even the hundredths of a degree, may be estimated, and thus it is possible to de- termine the height of a place by means of the boiling point to within about 10 feet. THERMO CHEMISTRY. All chemi- cal reactions are accompanied by changes in temperature. Exactly when this fact was first observed, it is impossible to say, but it must have been in the very early years of chemical investigation, for the phenomenon is so marked that it could not long escape notice. In the majority of cases, there is a rise in tem- perature, or, in other words, heat is evolved, but in some cases, the tempera- ture falls, or heat is absorbed. Reac- tions of the former type are known as exothermic, those of the latter as endo- thermic. The first law governing these thermal changes was formulated by La- voisier and Laplace, who stated, as the result of their investigations, that: The amount of heat which is required to de- compose a compound into its constituents is exactly equal to that which was evolved when the compound was formed froTn these constituents. A second law of even greater importance was discov- ered by Hess, who demonstrated that: The heat evolved in a chcTnical reaction is the same whether it takes place in one or in several stages. For instance, car- bon may be burned either to carbon mon- oxide or carbon dioxide, according to conditions. Carbon monoxide may be burned to carbon dioxide, so that in ei- ther case the final chemical change is from carbon to carbon dioxide. The above law (known as the "Constancy of the heat sum") is, as a matter of fact, a direct consequence of a wider law— the principle of the conservation of energy — but it was worked out by Hess experi- mentally. Of recent years important progress has been made in this branch of science as the result of researches carried out by Julius Thomsen of Copen- hagen, and Berthelot, the famous French chemist. Thomsen devised new appara- tus for making thermo-chemical determi- nations, and collected a mass of thermo- chemical data whose value can scarcely be over-estimated. Berthelot also made improvements in apparatus, and his dis- coveries have resulted in a great ad- vance in elucidating the theories under- lying thermo-chemical phenomena. Three of the fundamental principles which he laid down are: (1) The thermal change due to a chemical reaction depends (providing no external work is done) only on the con- dition of the system at the beginning and end of the reaction, and not on the inter- mediate conditions. (2) The heat evolved in a chemical re- action is a measure of the corresponding physical and chemical work. (3) Every chemical change which oc- curs without the addition of external en- ergy tends to produce that substance or system of substances the formation of which is accompanied by the evolu- tion of the maximum amount of heat. This generalization is found to be not true in every case, but it applies to so many instances that it is generally held