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 PHYSICS

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PHYSICS

no progress having been made along this line from the year 1300, when Thierry of Freiberg had given his treatise on it. However, the reason why the rays emerging from the drops of water are variously col- oured was no better known by Descartes than by Aristotle; it remained for Newton to make the dis- covery.

XXI. Progress of Experimental Physics. — Even in Descartes's work the discoveries in physics were almost independent of Cartesianism. The knowledge of natural truths continued to advance without the influence of this system and, at times, even in opposition to it, although those to whom this progress was due were often Cartesians. This ad- vancement was largely the result of a more frequent and skilful use of the experimental method. The art of making logically connected experiments and of deducing their consequences is indeed very ancient; in a way the works produced by this art were no more perfect than the researches of Pierre of Maricourt on the magnet or Thierry of Freiberg on the rainbow. However, if the art remained the same, its technic continued to improve; more skilled workmen and more powerful processes furnishing physicists with more intricate and better made instruments, and thus rendering possible more delicate experiments. The rather imperfect tests made by Galileo and Mersenne in endeavouring to determine the specific weight of air mark the beginning of the development of the experimental method, which was at once vigorously pushed forward by discussions in regard to vacuum.

In Peripatetic physics the possibility of an empty space was a logical contradiction; but, after the condemnation pronounced at Paris in 1277 by Tem- pier, the existence of a vacuum ceased to be consid- ered absurd. It was simply taught as a fact that the powers of nature are so constructed as to oppose the production of an empty space. Of the various conjectures proposed concerning the forces which prevent the appearance of a vacuum, the most sen- sible and, it would seem, the most generally received among sixteenth-century Parisians, was the follow- ing: contiguous bodies adhere to one another, and this adhe.sion is maintained by forces resembling those by which a piece of iron adheres to the magnet which it touches. In naming this force horror vacui, there was no intention of considering the bodies as animate beings. A heavy piece of iron detaches itself from the magnet that should hold it up, its weight having conquered the force by which the magnet retained it; in the same way, the weight of too heavy a body can prevent the horror vacui from raising this body. This very logical corollary of the hypothesis we have just mentioned was formulated by Galileo, who saw therein the explanation of a fact well-known to the cistern makers of his time; namely, that a suction-pump could not raise water higher than thirty-two feet. This corollary entailed the possibility of producing an empty space, a fact known to Torricelli who, in 1644, made the celebrated experi- ment with mercury that was destined to immortalize his name. However, at the same time, he anticipated a new explanation of this experiment; the mercury is supported in the tube not by the horror vacui that docs not exist, but by the pressure which the heavy air exerts on the exterior surface of the basin.

Torricelli's experiment quickly attracted the atten- tion of physicists. In France, thanks to Mersenne, it called forth on his part, and on that of those who had dealings with him, many experiments in which Roberval and Pascal (1623-62) vied with each other in ingenuity, and in order to have the resources of technic more easily at his disposal, Pascal made his startling ex^jcriments in a glass factory at Rouen. Among the mimcrous inquirers interested in Torri- celli's experiment some accepted the explanation offered by the "column of air", and advanced by the

great Italian geometrician himself; whereas others, such as Roberval, held to the ancient hypothesis of an attraction analogous to magnetic action. At length, with a view to settling the difference, an experiment was made which consisted in measuring at what height the mercury remained suspended in Torricelli's tube; observing it first of all at the foot of a mountain and then at its summit. The idea of this cx-periment seemed to have suggested itself to several physicists, notably Mersenne, Descartes, and Pascal and, through the instrumentality of the last named and the courtesy of P^rier, his brother-in- law, it was made between the base and summit of Puy-de-D6me, 19 Sept., 1648. The "Traite de 1' 6quilibre de liqueurs et de la pesanteur de la masse de I'air", which Pascal subsequently composed, is justly cited as a model of the art of logically connected experiments with deductions. Between atomists and Cartesians there were many discussions as to whether the upper part of Torricelli's tube was really empty or filled with subtile matter; but these dis- cussions bore little fruit. However, fortunately for physics, the experimental method so accurately fol- lowed by Torricelli, Pascal, and their rivals continued to progress.

Otto von Guericke (1602-86) seems to have pre- ceded TorricelU in the production of an empty space, since, between 1632 and 1638, he appears to have constructed his first pneumatic machine, with the aid of which instrument he made in 1654 the celebrated Magdeburg ex-pcriments, published in 1657 by his friend Caspar Schoot, S.J. (1608-60). Informed by Schoot of Guericke's researches, Robert Boyle (1627-91) perfected the pneumatic machine and, assisted by Richard Townley, his pupil, pursued the experiments that made known the law of the com- pressibility of perfect gases. In France these experi- ments were taken up and followed by Mariotte (1620-84). The use of the dilatation of a fluid for showing the changes of temperature w-as already known to Galileo, but it is uncertain whether the thermoscope was invented by Galileo or by some one of the numerous physicists to whom the priority is attributed, among these being Santorio, called Sanc- torius (1560-16:36), Fra Paolo Sarpi (1552-1623), Cornelis van Drebbel (1572-1634), and Robert Fludd (1574-1637). Although the various thermoscopes for air or hquid used in the very beginning admitted of only arbitrary graduation, they nevertheless served to indicate the constancy of the temperature or the direction of its variations, and consequently contrib- uted to the discovery of a number of the laws of physics. Hence this apparatus was used in the Accademia del Cimento, opened at Florence 19 June, 1657, and devoted to the study of experimental physics. To the members of this academy we are especially indebted for the demonstration of the constancy of the point of fusion of ice and of the absorption of heat accompanying this fusion. Observations of this kind, made by means of the thermoscope, created an ardent desire for the transformation of this appa- ratus into a thermometer, by the aid of a definite graduation so arranged that everywhere instruments could be made which would be comparable with one another. This problem, one of the most important in physics, was not solved until 1702 when Guillaume Amontons (1663-1705) worked it out in the most remarkable manner. Amontons took as a starting- point these two laws, discovered or verified by hinr the boiling point of water under atmospheric pressure is constant. The pressures sustained by any two masses of air, heated in the same way in any two con- stant volumes, have a relation independent of the temperature. These two laws enabled Amontons to use the air thermometer under constant volume and to graduate it in such a way that it gave what we to-day call absolute temperature. Of all the definj-