Popular Science Monthly/Volume 52/March 1898/The First Thermometers
|THE FIRST THERMOMETERS.|
THE thermometer, the Abbé Nollet writes, came for the first time from the hands of a peasant of North Holland. This peasant, whose name was Drebbel, was not, however, in fact, one of those coarse fellows who know of nothing but field work; he seems to have been of a diligent nature, and had apparently some knowledge of the physics of the time. An ingenious inventor as well as an impudent pretender, and boasting that he had discovered perpetual motion, while he made great advances in the art of dyeing cloths, he secured favors from James I; Rodolf II gave him liberal pensions and brought him to his court; and Ferdinand II, who was himself interested in the thermometer, chose him as the tutor for his son.
Drebbel's thermometer—an invention which he may have borrowed from Porta, and in which Galileo doubtless preceded him—was composed of a vertical glass tube ending at the top in a bulb, while the lower end was plunged into a vessel filled with water or some colored liquid. When the bulb was warmed, a part of the air contained within it was driven back into the water and escaped without. When the air became cool again as the temperature around it, the external pressure caused the liquid to rise in the tube, the limit of its ascent being determined by the degree to which the air in the bulb had been heated, and the tension it had acquired.
This hardly practical apparatus was still used in Germany as late as 1621. The members of the Accademia del Cimento, with their active interest in all physical progress, soon substituted for it the more convenient instrument which we still use. Contained in a transparent bulb prolonged into a fine tube, a liquid more dilatable than the bulb rose in the tube when it was warmed, and descended when it was cooled. The Florentine Academy, moreover, never let any physical discovery pass without trying to apply it to the healing art. Galileo had hardly recognized the constancy of the time of the oscillations of the pendulum before the pendulum was used to determine the rapidity or the slowness of the pulses of patients. The thermometer, made convenient and portable, became in the hands of the Venetian physiologist Santorio Santori a sensitive and precise indicator of the progress of fever. Santori's writings made the instrument popular, and it was soon common in the enamelers' shops as the Florence or Sanctorius thermometer.
It is hard to imagine the interest that was excited by the indications of this instrument, which was declared to be "worthy of Archimedes." Everybody was curious to observe the ascent or descent of the colored spirit in the tube; for, Nollet wrote, "the physician, guided by the thermometer, can labor with more certainty and success; the good citizen is better informed regarding the variations that concern the health of men and the productions of the earth; and the individual who is trying to procure the conveniences of life is informed by it as to what he must do in order to live all the year in a nearly uniform temperature." According to Amontons, Colbert had a project for constructing a large number of thermometers and sending them to different parts of the earth for making observations on seasons and climates, but was obliged to give it up on account of the imperfect character of the spirit thermometer of the time. Different instruments would not agree.
The marking of the degrees on the thermometer stems was not controlled by any fixed rule, and they therefore did not express the same heat or the same cold by the same number of degrees. To remedy this defect, some physicists advised that the lowest point reached in the extreme cold of winter and the highest in summer be marked, and the space between be divided into a hundred equal parts. Such a thermometer would indeed permit its owner to compare the cold and heat of different years; but in communicating his observations to another he would give him data that would have no meaning unless he also sent him the instrument he had used, or one having identical graduations.
The problem was first solved in 1702 by Amontons; and his method, although it has been given up and resumed at intervals, has now become the normal one to which all others are subordinated. It is based upon two observations, both of which are of primary importance. We take two masses of air in two bulbs. Each of these masses is separated from the outer air by a curved tube filled with mercury, forming a manometer. Suppose that at a given temperature one of these masses supports a pressure of one, and the other of two atmospheres. Warm the two masses of air equally, and pour into both manometers enough mercury to maintain invariable the volume occupied by each of them. While the pressure supported by the first mass will increase to a certain amount, that sustained by the other mass will increase doubly. The pressure on the second will always be double that on the first. Thus, when we warm the two masses equally, while keeping invariable the volume of the recipients containing them, a constant relation will be maintained between the pressures supported by them. This is Amontons's first observation.
In the second observation, which can be made with an arbitrarily graduated thermometer, the temperature of boiling water is found to be invariable. Not only does the thermometer immersed in water keep for any number of hours of boiling the height it had reached when the first bubbles came up, but it ascends to the same point every time it is placed in boiling water. If Amontons had added the proviso that the pressure of the atmosphere should be the same in all the experiments, which we know now is indispensable, he would have been rigorously exact.
When we take a bulb of air connected with a manometer, mark carefully the pressure which it sustains when it is plunged into boiling water, and then the pressure at which, under other circumstances, it reaches the same volume, the ratio of that pressure to the former may be regarded as expressing the ratio between the temperature to which the air was raised under the latter condition to the fixed temperature of boiling water. This ratio will be the same, whatever thermometer, constructed in the same way, we may use. In this way we have a sure means of obtaining instruments that can be compared with one another.
Amontons proposed for a thermometer, as Drebbel did, a mass of air maintained at a constant volume under a variable pressure. The rule by which he attached a certain degree of temperature to each degree of heat and cold, or a larger number for more intense heat and a smaller for cold, is the same rule to which Desormes and Clément on the one hand, and Laplace on the other, returned a century afterward; and is the rule proposed in the works of Sadi Carnot, Clausius, and Lord Kelvin as the measure of the absolute temperature.
The profound reasons which cause us to prefer the definition of temperature proposed by Amontons to every other could not be divined at the beginning of the eighteenth century. The large size and inconvenient shape of Amontons's instrument, and the necessity of taking account of the variations of atmospheric pressure in interpreting its indications, prevented its general adoption; and the Florence thermometer was still preferred. Spirit thermometers, that could be compared with one another, were in demand. Réaumur furnished them.
Réaumur observed, in 1730, that a thermometer placed in freezing water went down to a certain degree, and remained fixed there as long as the water was not wholly solidified. The temperature of water in process of congelation was therefore always the same, and fixed. As physics has advanced, some corrections have been made in this law, and causes have been discovered that make the point of congelation of water vary; and physicists have been induced, in view of it, to take as their fixed temperature, instead of the freezing point of water, the melting point of it. But neither these corrections nor the incidental recognition by the Florentine Academicians of the invariability of the melting point of ice diminish the importance of Réaumur's discovery.
Having discovered a fixed temperature, Réaumur deduced a way of making spirit thermometers that could be compared with one another. If we plunge a glass bulb prolonged into a fine tube and filled with spirit into freezing water, and draw a line marked zero flush with the top of the liquid, then determine the volume occupied by the liquid under these conditions; if we divide the tube into portions, the interior capacity of which represents at the temperature of the freezing of water aliquot parts of that volume—hundredths, for example—and number these divisions from the line marked zero; then if, in an experiment, we see the spirit rise to the level of the division marked five, we know that the spirit in the glass has suffered an apparent dilatation of five hundredths between the freezing temperature of water and the temperature of the experiment. If we always take care to use spirit of the same quality—and Réaumur prescribed minute rules on this subject—and if we neglect the changes which the variable nature of the glass will introduce into the law of dilatation of the thermometric receptacle, we will obtain instruments of a kind that will always mark the same degree when they are equally heated or cooled.
For two instruments constructed according to the laws laid down by Réaumur to be rigorously comparable, it was essential that they be made of the same glass and filled with the same liquid. If the glass of which they are made has not exactly the same composition and tempering in both, and the alcohol has not the same degree of concentration, they will not agree. In order to diminish these variations, it is convenient to fix all thermometers, whatever they may be made of, so that they shall give the same indications for two fixed temperatures. The point reached by the liquid at the lower of these temperatures is marked on the instrument, and then it is raised to the higher temperature, and the point which it reaches then is marked. The interval is then divided into parts having the same interior volume, and the division is carried out beyond the fixed points. In such thermometers the liquid will stand at the same mark for an equal degree of heat, notwithstanding slight inequalities in the glass and the fluid.
It was some time before the two fixed temperatures at which the thermometric scale should be marked were determined upon. Dalence, in 1688, took a mixture of water and ice for the zero, and the melting point of butter as the upper point. Renaldini, in 1694, recommended a mixture of water and ice and the boiling point of water, but his process was not applicable to the alcohol thermometers then in use, for the vapor of alcohol has a tension at the boiling point of water which would burst the reservoirs of the instruments. And Renaldini's method could not be adopted till after Musschenbroeck had introduced the use of mercury. In 1729, Delisle chose as graduating points the temperature of ice-water and the almost invariable temperature of the cellars of the Observatory at Paris.
About 1714 a skillful instrument-maker of Dantsic, Daniel Gabriel Fahrenheit, furnished chemists with alcohol thermometers which he replaced in 1720 with mercury thermometers, the indications given by which all agreed with one another. According to the chemist Woulfe, he boasted that he could make a thermometer that would agree with those he had already made, in any place, and without seeing any of the instruments that had already gone out of his hands; but he would not divulge the process by which he had been able to obtain such an agreement. This process, in establishing which he had been aided by the advice of the astronomer Roemer, was nothing else than the method devised by Dalencé; but Fahrenheit took for his zero the temperature of a mixture of ice and muriate of ammonia (chloride of ammonium)—which, he thought, was the greatest cold that could be obtained—and for his higher degree the temperature of the human body.
Finally, in 1742, the Swede, Andrew Celsius, proposed to restore the method of Renaldini, and divide into a hundred degrees the interval which the mercury in the thermometer would traverse between the temperature of melting ice and that of boiling water. He marked the lower temperature 100, and the higher 0. Linnæus, reversing this order, gave the mercury thermometer (centigrade) the form under which it is now known.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.