TERMOLOGIA

THERMOLOGY. — It is the branch of physics that systematically studies phenomena in which heat plays a predominant role. Everyone has some notion of heat linked to the physiological sensations of hot and cold, or of more or less hot, or more or less cold. From ancient times, this vaguely conceived notion of heat has been associated—and often confused—with temperature, an error that still occurs not infrequently. It is certain, however, that among physicists the two notions of heat, or rather of quantity of heat and temperature, had already been completely disentangled by the first half of the 18th century. The criteria for measuring these quantities are therefore of fundamental importance.

I. MEASUREMENT OF TEMPERATURE

First, it must be observed that the characteristic that sharply distinguishes temperature from the usual physical quantities—lengths, masses, forces, etc.—lies in the fact that while each of these is measurable by means of a given standard, for the measurement of temperature an entire scale must be established. It was precisely this that Galileo and later the Accademia del Cimento had practically accomplished, more than a century before the clarification of the notion of temperature, with their noted constructions of the first thermometers.

Today, physics has at its disposal many criteria for measuring temperature, but the simplest and still most widely used is that of exploiting the much greater thermal expansion of fluids compared to solid bodies. Galileo and his early disciples used air as the expanding fluid, which was later advantageously replaced by liquid fluids. The devices that soon followed took on the general form of today’s mercury thermometers. To reduce the arbitrariness of the scale, it was agreed as early as 1694 (Renaldini) to designate as fixed temperatures those of melting ice and boiling water, which are still in use today, dividing the interval into a certain number of parts. The scales known as Réaumur’s and Celsius’s (or centigrade) correspond respectively to dividing the interval into 80 and 100 degrees.

These scales, once defined, did not remain rigorously identical when the liquid (mercury, toluene, alcohol, etc.) used was changed. It was therefore agreed that, for scientific purposes, one should always refer to the scale of the gas thermometer constructed according to specific standards and precise calibration. This scale was called the absolute scale, and also, for its particular agreement with the thermodynamic viewpoint, the thermodynamic scale. Its lowest temperature, absolute zero, corresponds to –273 degrees on the centigrade scale; and since in it the interval between the temperatures of melting ice and boiling water is divided into 100 parts, it follows that the values T° of the absolute temperature are expressed by the formula T° = t° + 273.

II. MEASUREMENT OF QUANTITIES OF HEAT

At the basis of this fundamental measurement lies the primitive common notion that in any body the addition or removal of heat must always produce a corresponding increase or decrease in its temperature. To make this not entirely exact notion less vague, it is generally added that the addition or removal of heat should produce no other change in the body in question besides the variation in temperature. If, in addition, the condition is imposed that the temperature variations should be small, experience confirms not only the qualitative reliability of the notion but also demonstrates the proportionality between heat and temperature variation.

Naturally, for the measurement of quantities of heat it is necessary to define arbitrarily but precisely a unit; and for this the calorie (or large calorie or kilocalorie) was chosen, namely the quantity of heat required to raise the temperature of one kilogram of water from 14.5° to 15.5° C., usually denoted by Cal.

Evidently, such measurements cannot be direct, as, for example, those of length by means of the meter unit, but must necessarily be indirect. The numerous devices devised for such measurements under the most varied conditions are called calorimeters.

III. SPECIFIC HEATS

A calorie introduced into one kilogram of water raises its temperature by one degree, but introduced into one kilogram of other substances it produces even greater increases, with very rare exceptions; for example, introduced into one kilogram of copper it raises the temperature by more than 10°, and into one kilogram of mercury by 30°. This necessitates assigning to each substance a so-called specific heat, which physically is equivalent to the quantity of heat required to raise the temperature of the unit mass of that substance by one degree Celsius. It follows that the specific heat of water is equal to 1, that of copper to 0.095, that of mercury to 0.033, etc.

The specific heats of the elements, multiplied by their atomic weights (called atomic heats), are of great interest to modern chemistry and physics.

TERMINOLOGY — TERNI AND NARNI

meters in many industrial processes, especially at high temperatures.

V. THERMAL ENGINES

These can be distinguished into machines that use heat, produced in various ways, for the production of mechanical energy, and refrigerating machines.

Among the former are stationary and mobile steam engines and steam turbines, which are employed to develop very great power; gasoline internal combustion engines, widely used in automobiles; the more powerful heavy-oil internal combustion engines, also known as Diesel engines; and the recent and extremely powerful jet engines used in military and civil aviation, which permit speeds exceeding that of sound.

Refrigerating machines, which were initially limited to large industrial plants, are now widely penetrating domestic life. All operate on the basis of the fundamental principles of thermodynamics (v.), for whose rational and practical application a vast technology has now been established.

TERNI AND NARNI, DIOCESE OF

Exterior of the church of S. Francesco (13th cent.).

IV. THERMAL PHENOMENA AND THEIR APPLICATIONS

Given their great number, it is not possible to do more than touch upon them very briefly. Changes of state: from the solid to the liquid state: fusion, always accompanied by a considerable absorption of heat; e.g., the fusion of one kilogram of ice requires about 80 Cal.; from the liquid to the solid state: solidification, always accompanied by the inverse liberation of heat (latent in the liquid); from the liquid to the aeriform state: evaporation, always accompanied by heat absorptions, sometimes considerable; e.g., the change of 1 kg of water at 100° into 1 kg of steam at the same temperature requires 537 Cal.; inversely, with condensation, accompanied by the liberation of the equivalent heat (latent in the aeriform state). It is interesting to gain a clear understanding of the phenomena of evaporation and condensation because many important applications are based upon them. During evaporation, i.e., as long as the vapour remains in contact with some part of its liquid, the vapour is said to be saturated and has particular characteristics which distinguish it from gases: in particular, its pressure depends only on its temperature, not on its volume; if an attempt is made to compress it, it condenses. But as soon as the liquid has completely evaporated, the said vapour behaves like a gas and, like it, approximately follows the well-known Gay-Lussac law characteristic of gases: pressure × volume = R × absolute temperature, with R constant. Moreover, it should be borne in mind that for every substance there exists a critical temperature above which it cannot exist in the condensed state no matter how high the pressure to which it is subjected. This temperature for water is 374° C, for carbon dioxide 31.2° (which is why it is dangerous to store cylinders of this substance in rooms that may reach that temperature), for oxygen —116°, for hydrogen —241°. It is this knowledge which, after so many vain attempts, has made it possible to liquefy all gases, even those which were said to be permanent, by the combined action of pressure and cooling.

Finally, it is worth recalling that there exist thermoelectric phenomena due to interactions of heat and electricity.

Of great importance are the decrease, with temperature, of the electrical resistance of metallic conductors and the analogous variation of the electromotive forces which always manifest themselves at the contact of different metals. These two categories of phenomena have been widely utilized, in particular for the measurement of temperatures, which can be obtained with devices of adjustable sensitivity at will, more convenient and easier to use than ordinary thermometers. They are now used not only for scientific purposes, but have also replaced thermo