Heroes of Science: Physicists. Garnett William

discussed by Lord Bacon, but at best his experimental methods, though most complete and systematic, existed only upon paper, and it was reserved for Boyle and his friends to put the Baconian philosophy into actual practice.

      It was during his residence at Oxford that he invented the air-pump, which was afterwards improved for him by Hooke, and with which he conducted most of those experiments on the "spring" and weight of the air, which led up to the investigations that have rendered his name inseparably connected with "the gaseous laws." The experiments of Galileo and of Torricelli had shown that the pressure of the air was capable of supporting a column of water about thirty-four feet in height, or a column of mercury nearly thirty inches high. The younger Pascal, at the request of Torricelli, had carried a barometer to the summit of the Puy de Dome, and demonstrated that the height of the column of mercury supported by the air diminishes as the altitude is increased. Otto von Guericke had constructed the Magdeburg hemispheres, and shown that, when exhausted, they could not be separated by sixteen horses, eight pulling one way and eight the other. He was aware that the same traction could have been produced by eight horses if one of the hemispheres had been attached to a fixed obstacle; but, with the instincts of a popular lecturer, he considered that the spectacle would thus be rendered less striking, and it was prepared for the king's entertainment. Boyle wished for an air-pump with an aperture in the receiver sufficiently large for the introduction of various objects, and an arrangement for exhausting it without filling the receiver with water or otherwise interfering with the objects placed therein. His apparatus consisted of a large glass globe capable of containing about three gallons or thereabouts, terminating in an open tube below, and with an aperture of about four inches diameter at the top. Around this aperture was cemented a turned brass ring, the inner surface being conical, and into this conical seat was fitted a brass plate with a thick rim, but drilled with a small hole in the centre. To this hole, which was also conical, was fitted a brass stopper, which could be turned round when the receiver was exhausted. By attaching a string to this stopper, which was so long as to enter the receiver to the depth of two or three inches, and turning the stopper in its seat, the string could be wound up, and thus objects could be moved within the receiver. The tube at the bottom of the receiver communicated with a stop-cock, and this with the upper end of the pumpbarrel, which was inverted, so that this stop-cock, which was at the top of the barrel, took the place of the foot-valve. The piston was solid, made of wood, and surrounded with sole leather, which was kept well greased. There being no valve in the piston, it was necessary to place an exhaust-valve in the upper end of the cylinder. This consisted of a small brass plug closing a conical hole so that it could be removed at pleasure. The construction of the cylinder was, therefore, similar to that of an ordinary force-pump, except that the valves had to be moved by hand (as in the early forms of the steam-engine). The piston was raised and depressed by means of a rack and pinion. The pumps could be used either for exhausting the receiver or for forcing air into it, according to the order in which the "valves" were opened. If the stop-cock communicating with the receiver were open while the piston was being drawn down, and the brass plug removed so as to open the exhaust-valve when the piston was being forced up, the receiver would gradually be exhausted. If the brass plug were removed during the descent of the piston, and the stop-cock opened during its ascent, air would be forced into the receiver. In the latter case it was necessary to take special precautions to prevent the brass plate at the top of the receiver being raised from its seat. All joints were made air-tight with "diachylon," and when, through the bursting of a glass bulb within it, the receiver became cracked, the crack was rendered air-tight by the same means. Other receivers of smaller capacity were also provided, on account of the greater readiness with which they could be exhausted.

      With this apparatus Boyle carried out a long series of experiments. He could reduce the pressure in the large receiver to somewhat less than that corresponding to an inch of mercury, or about a foot of water. Squeezing a bladder so as to expel nearly all the air, tying the neck, and then introducing it into the receiver, he found, on working the pump, that the bladder swelled so that at length it became completely distended. In order to account for this great expansibility, Boyle pictured the constitution of the air in the following way. He supposed the air to consist of separate particles, each resembling a spiral spring, which became tightly wound when exposed to great pressure, but which expanded so as to occupy a larger circle when the pressure was diminished. Each of these little spirals he supposed to rotate about a diameter so as to exclude every other body from the sphere in which it moved. Increasing the length of the diameter tenfold would increase the volume of one of these spheres, and therefore the volume of the gas, a thousandfold. Possibly this was only intended as a mental illustration, exhibiting a mechanism by which very great expansion might conceivably be produced, and scarcely pretending to be considered a theory of the constitution of the air. Boyle's first idea seems to have been derived from a lock of wool in which the elasticity of each fibre caused the lock to expand after it had been compressed in the hand. In another passage he speaks of the air as consisting of a number of bodies capable of striking against a surface exposed to them. He demonstrated the weight of the air by placing a delicate balance within the receiver, suspending from one arm a bladder half filled with water, and balancing it with brass weights. On exhausting the air, the bladder preponderated, and, by repeating the experiment with additional weights on the other arm until a balance was effected in the exhausted receiver, he determined the amount of the preponderance. In another experiment he compressed air in a bladder by tying a pack-thread round it, balanced it from one arm of his balance in the open air; then, pricking the bladder so as to relieve the pressure, he found that with the escape of the compressed air the weight diminished.

      One of the most important of his experiments with the air-pump was the following. He placed within the receiver the cistern of a mercurial barometer, the tube of which was made to pass through the central hole in the brass plate, from which the stopper had been removed. The space around the tube was filled up with cement, and the receiver exhausted. At each stroke of the pump the mercury in the barometer tube descended, but through successively diminishing distances, until at length it stood only an inch above the mercury in the cistern. The experiment was then repeated with a tube four feet long and filled with water. This constituted the nineteenth experiment referred to later on. A great many strokes of the pump had to be made before the water began to descend. At length it fell till the surface in the tube stood only about a foot above that in the tank. Placing vessels of ordinary spring-water and of distilled rain-water in the receiver, he found that, after the exhaustion had reached a certain stage, bubbles of gas were copiously evolved from the spring-water, but not from the distilled water. On another occasion he caused warm water to boil by a few strokes of the pump; and, continuing the exhaustion, the water was made to boil at intervals until it became only lukewarm. The experiment was repeated with several volatile liquids. He also noticed the cloud formed in the receiver when the air was allowed rapidly to expand; but the mechanical theory of heat had not then made sufficient progress to enable him to account for the condensation by the loss of heat due to the work done by the expanding air. The very minute accuracy of his observations is conspicuous in the descriptions of most of his experiments. That the air is the usual medium for the conveyance of sound was shown by suspending a watch by a linen thread within the receiver. On exhausting the air, the ticking of the watch ceased to be heard. A pretty experiment consisted in placing a bottle of a certain fuming liquid within the receiver; on exhausting the air, the fumes fell over the neck of the bottle and poured over the stand on which it was placed like a stream of water. Another experiment, the thirty-second, is worthy of mention on account of the use to which it was afterwards applied in the controversy respecting the cause of suction. The receiver, having been exhausted, was removed from the cylinder, the stop-cock being turned off, and a small brass valve, to which a scale-pan was attached, was placed just under the aperture of the tube below the stop-cock. On turning the latter, the stream of air raised the valve, closing the aperture, and the atmospheric pressure supported it until a considerable weight had been placed in the scale-pan. Because the receiver could not be exhausted so thoroughly as the pump-cylinder, Boyle attempted to measure the pressure of the air by determining what weight could be supported by the piston. He found first that a weight of twenty-eight pounds suspended directly from the piston was sufficient to overcome friction when air was admitted above the piston. When the access of air to the top of the piston was prevented, more than one hundred pounds additional weight was required to draw down the piston. The diameter of