The opposition of "hot" and "cold," like that of "dry and "moist, ' is an inevitable by-product of our sense of touch. These antitheses, noted by the pre-Socratic philosophers of Greek antiquity, were used by Aristotle in the formation of his doctrine of opposites, while from their combinations the four elementsearth, water, air, and fire--were built up. In doing this, Aristotle made no attempt to assign numbers to these qualities. The great physician Galen seems to have introduced the idea of "degrees of heat and cold," four in number each way from a neutral point in the middle. The neutral point was to be a mixcure of equal quantities of ice and boiling water, substances that Galen seems to have thought of as the hottest and coldest of materials. Whether the quantities were weights or volumes is not certain, and we may wonder whether Galen ever made such an experiment;? but it is the earliest notion of a fixed point or standard of temperature.
Strange as it may seem, the idea of a scale of temperature was familiar to physicians before they had any instrument to measure it with. This is illustrated by the De logistica medica of Johannis Hasler of Berne. Hasler's very first "Problem" is entitled "To find the natural degree of temperature of each man, as determined by his age, the time of year, the elevation of the pole [i.e., the latitude] and other influences." It was believed that the body temperature of dwellers in the tropics was higher than those living in higher latitudes. Hasler showed this supposed relationship by an elaborate table' (Fig. 1.1), in which the nine degrees of heat in the first column and the Galenic degrees of heat and cold in the second (divided into three parts in the fourth and third columns respectively) are set opposite the latitude. From this table the physician could read the normal degree of heat or cold to be expected in an inhabitant of any place and thus decide how to mix his medicines.
This was the medical scale. There was also a "philosophical scale" with eight degrees of heat and eight of cold. As we shall see, the first thermometer of which we have a description and illustration has a scale of "degrees of cold that goes from one to eight.
Questions of priority are loaded with embarrassment for the historian of science and technology, even if they are of great interest to the general reader. The thermometer provides a particularly acute example, at least partly because more than a piece of apparatus is involved. Its "invention" cannot be considered apart from its use and calibration. According to the point of view adopted in this chapter a distinction must be made between the terms thermoscope and thermometer, in which a thermometer is simply a thermoscope provided with a scale. This may seem too elementary to be worth notice; but if it had been kept in mind, many gallons of ink might have been saved in the attempt to establish when, where, and by whom "the thermometer was invented." I propose to regard it as axiomatic that a "meter" ' must have a scale or something equivalent. If this is admitted, the problem of the invention of the thermometer becomes more straightforward; that of the invention of the thermoscope remains as obscure as ever.
As to the thermoscope, a further preliminary question must be answered: when does a pneumatic experiment, of whatever sort, become a thermoscope? I take it as essential that the experi menter should have had clearly in view the construction of an instrument intended to give some visible indication of changes in its condition with respect to heat. It is not enough that the behavior of some instrument should be interbretable in this way.
Pneumatic experiments that could form the basis of a thermoscope were made in antiquity by Philo of Byzantium, who probably flourished about the end of the second century B.C.,' and also by Hero of Alexandria, possibly in the first half of the first century B.C, but perhaps much later. Philo's work, lost in the original Greek, remained in Latin and Arabic manuscripts, unpublished, until the end of the nineteenth century.• Hero's Pneumatics fared better, having been published in Latin in 1575,10 in an Italian translation in 1589,11 and again in Italian in 1592,12 the year in which Galileo took up his post at Padua.
Hero's work was studied a great deal in Italy toward the end of the sixteenth century, as Hellmann has found.13 Galileo is known to have read it by 1594.14 Meanwhile Giambattista della Porta had read about Hero's experiment, of a "fountain that drips in the sun," and described an apparatus that could have been used as an air thermoscope but was in fact intended only to show that water could be raised by the action of heat.15 Even in 1606 it is plain that a similar experiment described by Della Porta merely shows the expansion of air by heat and its contraction by cold, but that he had no idea of making a measuring instrument.1 Like many others, he was repeating some of Hero's experiments, with variations.
The serious candidates for the honor of having "invented the thermometer" are usually considered to be four in number: Galileo, Santorio (or Sanctorius), Drebbel, and Fludd. The first two lived in Italy, the other two lived north of the Alps. As interaction across the Alps is very unlikely in this case, we may profitably consider the two pairs separately.
Like the British idolators of Newton, the Italian parti. sans of Galileo have done their best to magnify the achievements of their hero, often with scant regard for historical method, or even for probability. It may now, I think, be maintained categorically that Santorio applied a measuring device to the air thermoscope, at least as early as 1612, thus making an air thermometer; although he was not the only one to have done this, as we shall see. Whether he or Galileo, or someone else, first made an air thermoscope is much less clear. I suspect that if the great name of Galileo were not involved it would not really be of much importance, in view of the very small utility of the thermoscope.
Three pieces of documentary evidence are usually cited in support of Galileo's claim to the invention. The first consists of a series of letters from Giovanfrancesco Sagredo to Galileo in the period June, 1612, to April, 1615. The second is a letter from Benedetto Castelli to Ferdinando Cesarini, written in 1638. The third is Vincenzio Viviani's biography of Galileo, written in 1654.
On June 30, 1612, Sagredo wrote a letter to Galileo in which the following passage occurs:
Signor Mula . . . told me about an instrument of Santorio's, with with which cold and heat were measured by means of compasses;17 and finally let me know that this is a large glass bulbs with a long neck. I immediately devoted myself to making some very fine and elegant ones.19
Sagredo was apparently much taken with this new invention, for he goes on:
I make the ordinary ones at a cost of four lire each, for a wine glass with a foot, a small ampoule, and a glass tube; and I work so fast that in an hour I finish as many as ten of them. The finest that I have made was worked at the lamp, and it is in all its parts as in the enclosed scale drawing.20
It may be supposed that the ordinary ones were assembled with mastic and perhaps a cork, in view of the reference to glass blowing in the description of the finest one.
Galileo's reply to this has not been preserved, but within the next few months Sagredo had been told that this was an invention made by the great philosopher, for on May 9, 1613, we find him writing again:
forms,22 so that the difference in temperature between one room and another is seen to be as much as 100 degrees. With these I have found various marvellous things, as, for example, that in winter the air may be colder than ice or snow; that the water just now appears colder than the air; that small bodies of water are colder than large ones, and similar subtle matters.
He goes on to say that the Aristotelian philosopher Bernardino Gaio cannot understand how it works, thinking that the "attractive virtue of heat should make the liquid rise farther in the tube when the bulb is hotter.