Galileo, to whom the advance in exact science is so largely indebted, must also be credited with the first apparatus for the measurement of temperatures. This was invented before 1603 and consisted of a glass bulb with a long stem of the thickness of a straw. The bulb was first heated and the stem placed in water. The point at which the water, which rose in the tube, might stand was an indication of the temperature. In 1631 Jean Rey just inverted this contrivance, filling the bulb with water. Of course these thermoscopes would register the effect of varying pressures as well as temperatures, and they soon made way for the thermometer and the barometer. Before 1641 a true thermometer was constructed by sealing the top of the tube after driving out the air by heat. Spirits of wine were used in place of water. Mercury was not employed till 1670.
Descartes and Galileo had brought under criticism the ancient idea that nature abhors a vacuum. They knew that the horror vacui was not sufficient to raise water in a pump more than about thirty-three feet. They had also known that air has weight, a fact which soon served to explain the so-called force of suction. Galileo's associate Torricelli reasoned that if the pressure of the air was sufficient to support a column of water thirty-three feet in height, it would support a column of mercury of equal weight. Accordingly in 1643 he made the experiment of filling with mercury a glass tube four feet long closed at the upper end, and then opening the lower end in a basin of mercury. The mercury in the tube sank until its level was about thirty inches above that of the mercury in the basin, leaving a vacuum in the upper part of the tube. As the specific gravity of mercury is 13, Torricelli knew that his supposition had been correct and that the column of mercury in the tube and the column of water in the pump were owing to the pressure or weight of the air.
Pascal thought that this pressure would be less at a high altitude. His supposition was tested on a church steeple at Paris, and, later, on the Puy de Dôme, a mountain in Auvergne. In the latter case a difference of three inches in the column of mercury was shown at the summit and base of the ascent. Later Pascal experimented with the siphon and succeeded in explaining it on the principle of atmospheric pressure.
Torricelli in the space at the top of his barometer (pressure-gauge) had produced what is called a Torricellian vacuum. Otto von Guericke, a burgomaster of Magdeburg, who had traveled in France and Italy, succeeded in constructing an air-pump by means of which air might be exhausted from a vessel. Some of his results became widely known in 1657, though his works were not published till 1673.
Robert Boyle (1626-1691), born at Castle Lismore in Ireland, was the seventh son and fourteenth child of the distinguished first Earl of Cork. He was early acquainted with these various experiments in reference to the air, as well as with Descartes' theory that air is nothing but a congeries or heap of small, and, for the most part, flexible particles. In 1659 he wrote his New Experiments Physico-Mechanical touching the Spring of the Air. Instead of spring, he at times used the word elater (ἐλατὴρ). In this treatise he describes experiments with the improved air-pump constructed at his suggestion by his assistant, Robert Hooke.
One of Boyle's critics, a professor at Louvain, while admitting that air had weight and elasticity, denied that these were sufficient to account for the results ascribed to them. Boyle thereupon published a Defence of the Doctrine touching the Spring and Weight of the Air. He felt able to prove that the elasticity of the air could under circumstances do far more than sustain twenty-nine or thirty inches of mercury. In support of his view he cited a recent experiment.
He had taken a piece of strong glass tubing fully twelve feet in length. (The experiment was made by a well-lighted staircase, the tube being suspended by strings.) The glass was heated more than a foot from the lower end, and bent so that the shorter leg of twelve inches was parallel with the longer. The former was hermetically sealed at the top and marked off in forty-eight quarter-inch spaces. Into the opening of the longer leg, also graduated, mercury was poured. At first only enough was introduced to fill the arch, or bent part of the tube below the graduated legs. The tube was then inclined so that the air might pass from one leg to the other, and equality of pressure at the start be assured. Then more mercury was introduced and every time that the air in the shorter leg was compressed a half or a quarter of an inch, a record was made of the height of the mercury in the long leg of the tube. Boyle reasoned that the compressed air was sustaining the pressure of the column of mercury in the long leg plus the pressure of the atmosphere at the tube's opening, equivalent to 292⁄16 inches of mercury. Some of the results were as follows: When the air in the short tube was compressed from 12 to 3 inches, it was under a pressure of 1179⁄16 inches of mercury; when compressed to 4 it was under pressure of 8715⁄16 inches of mercury; when compressed to 6, 5813⁄16; to 9, 395⁄8. Of course, when at the beginning of the experiment there were 12 inches of air in the short tube, it was under the pressure of the atmosphere, equal to that of 292⁄16 inches of mercury. Boyle with characteristic caution was not inclined to draw too general a conclusion from his experiment. However, it was evident, making allowance for some slight irregularity in the experimental results, that air reduced under pressure to one half its original volume, doubles its resistance; and that if it is further reduced to one half,—for example, from six to three inches,—it has four times the resistance of common air. In fact, Boyle had sustained the hypothesis that supposes the pressures and expansions to be in reciprocal proportions.
REFERENCES
Sir Robert S. Ball, Great Astronomers.