Heroes of Science: Physicists - Garnett William

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cylinder was about three inches.

      

      Boyle's style of reasoning is well illustrated by the following from his paper on "The Spring of the Air:"—

      "In the next place, these experiments may teach us what to judge of the vulgar axiom received for so many ages as an undoubted truth in the peripatetick schools, that Nature abhors and flieth a vacuum, and that to such a degree that no human power (to go no higher) is able to make one in the universe; wherein heaven and earth would change places, and all its other bodies rather act contrary to their own nature than suffer it.... It will not easily, then, be intelligibly made out how hatred or aversation, which is a passion of the soul, can either for a vacuum or any other object be supposed to be in water, or such like inanimate body, which cannot be presumed to know when a vacuum would ensue, if they did not bestir themselves to prevent it; nor to be so generous as to act contrary to what is most conducive to their own particular preservation for the public good of the universe. As much, then, of intelligible and probable truth as is contained in this metaphorical expression seems to amount but to this—that by the wise Author of nature (who is justly said to have made all things in number, weight, and measure) the universe, and the parts of it, are so contrived that it is hard to make a vacuum in it, as if they studiously conspired to prevent it. And how far this itself may be granted deserves to be further considered.

      "For, in the next place, our experiments seem to teach that the supposed aversation of Nature to a vacuum is but accidental, or in consequence, partly of the weight and fluidity, or, at least, fluxility of the bodies here below; and partly, and perhaps principally, of the air, whose restless endeavour to expand itself every way makes it either rush in itself or compel the interposed bodies into all spaces where it finds no greater resistance than it can surmount. And that in those motions which are made ob fugam vacui (as the common phrase is), bodies act without such generosity and consideration as is wont to be ascribed to them, is apparent enough in our thirty-second experiment, where the torrent of air, that seemed to strive to get into the emptied receiver, did plainly prevent its own design, by so impelling the valve as to make it shut the only orifice the air was to get [in] at. And if afterwards either Nature or the internal air had a design the external air should be attracted, they seemed to prosecute it very unwisely by continuing to suck the valve so strongly, when they found that by that suction the valve itself could not be drawn in; whereas, by forbearing to suck, the valve would, by its own weight, have fallen down and suffered the excluded air to return freely, and to fill again the exhausted vessel....

      "And as for the care of the public good of the universe ascribed to dead and stupid bodies, we shall only demand why, in our nineteenth experiment, upon the exsuction of the ambient air, the water deserted the upper half of the glass tube, and did not ascend to fill it up till the external air was let in upon it. Whereas, by its easy and sudden rejoining that upper part of the tube, it appeared both that there was then much space devoid of air, and that the water might, with small or no resistance, have ascended into it, if it could have done so without the impulsion of the readmitted air; which, it seems, was necessary to mind the water of its formerly neglected duty to the universe."

      Boyle then goes on to explain the phenomena correctly by the pressure of the air. Elsewhere he accounts for the diminished pressure on the top of a mountain by the diminished weight of the superincumbent column of air.

      The treatise on "The Spring of the Air" met with much opposition, and Boyle considered it necessary to defend his doctrine against the objections of Franciscus Linus and Hobbes. In this defence he described the experiment in connection with which he is most generally remembered. Linus had admitted that the air might possess a certain small amount of elasticity, but maintained that the force with which mercury rose in a barometer tube was due mainly to a totally different action, as though a string were pulling upon it from above. This was his funicular hypothesis. Boyle undertook to show that the pressure of the air might be made to support a much higher column of mercury than that of the barometer. To this end he took a glass tube several feet in length, and bent so as to form two vertical legs connected below. The shorter leg was little more than a foot long, and hermetically closed at the top. The longer leg was nearly eight feet in length, and open at the top. The tube was suspended by strings upon the staircase, the bend at the bottom pressing lightly against the bottom of a box placed to receive the mercury employed in case of accident. Each leg of the tube was provided with a paper scale. Mercury was poured in at the open end, the tube being tilted so as to allow some of the air to escape from the shorter limb until the mercury stood at the same level in both legs when the tube was vertical. The length of the closed tube occupied by the air was then just twelve inches. The height of the barometer was about 29-1/8 inches. Mercury was gently poured into the open limb by one operator, while another watched its height in the closed limb. The results of the experiments are given in the table on the opposite page.

      In this table the third column gives the result of adding to the second column the height of the barometer, which expresses in inches of mercury the pressure of the air on the free surface of the mercury in the longer limb. The fourth column gives the total pressure, in inches of mercury, on the hypothesis that the pressure of the air varies inversely as the volume. The agreement between the third and fourth columns is very close, considering the roughness of the experiment and that no trouble appears to have been taken to calibrate the shorter limb of the tube, and justified Boyle in concluding that the hypothesis referred to expresses the relation between the volume and pressure of a given mass of air.

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Length of closed tube occupied by air. Height of mercury in open tube above that in closed tube. Total pressure on air in inches of mercury. Total pressure according to Boyle's law.
12 0 29-2/16 29-2/16
11-1/2 1-7/16 30-9/16 30-6/16
11 2-13/16 31-15/16 31-12/16
10-1/2 4-6/16 33-8/16 33-1/7
10 6-3/16 35-5/16 35
9-1/2 7-14/16 37 36-15/19
9 10-1/16 39-3/16 38-7/8
8-1/2 12-8/16 41-10/16 41-2/17
8 15-1/16 44-3/16 43-11/16
7-1/2 17-15/16 47-1/16 46-3/5
7 21-3/16 50-5/16 50
6-1/2 25-3/16 54-5/16 53-10/13
6 29-11/16 58-13/16 58-2/8