Heroes of Science: Physicists. Garnett William
To extend the investigation so as to include expansion below atmospheric pressure, a different apparatus was employed. It consisted of a glass tube about six feet in length, closed at the lower end and filled with mercury. Into this bath of mercury was plunged a length of quill tube, and the upper end was sealed with wax. When the wax and air in the tube had cooled, a hot pin was passed through the wax, making a small orifice by which the amount of air in the tube was adjusted so as to occupy exactly one inch of its length as measured by a paper scale attached thereto, after again sealing the wax. The quill tube was then raised, and the height of the surface of the mercury in the tube above that in the bath noticed, together with the length of the tube occupied by the air. The difference between the height of the barometer and the height of the mercury in the tube above that in the bath gave the pressure on the imprisoned air in inches of mercury. The result showed that the volume varied very nearly in the inverse ratio of the pressure. A certain amount of air, however, clung to the sides of the quill tube when immersed in the mercury, and no care was taken to remove it by boiling the mercury or otherwise; in consequence of this, as the mercury descended, this air escaped and joined the rest of the air in the tube. This made the pressure rather greater than it should have been towards the end of the experiment, and when the tube was again pressed down into the bath it was found that, when the surfaces of the mercury within and without the tube were at the same level, the air occupied nearly 1-1/8 inch instead of one inch of the tube. These experiments first established the truth of the great law known as "Boyle's law," which states that the volume of a given mass of a perfect gas varies inversely as the pressure to which it is exposed.
Another experiment, to show that the pressure of the air was the cause of suction, Boyle succeeded in carrying out at a later date. Two discs of marble were carefully polished, so that when a little spirit of turpentine was placed between them the lower disc, with a pound weight suspended from it, was supported by the upper one. The apparatus was introduced into the air-pump, and a considerable amount of shaking proved insufficient to separate the discs. After sixteen strokes of the pump, on opening the communication between the receiver and cylinder, when no mechanical vibration occurred, the discs separated.
Upon the Restoration in 1660, the Earl of Clarendon, who was Lord Chancellor of England, endeavoured to persuade Boyle to enter holy orders, urging the interest of the Church as the chief motive for the proceeding. This made some impression upon Boyle, but he declined for two reasons—first, because he thought that he would have a greater influence for good if he had no share in the patrimony of the Church; and next, because he had never felt "an inward motion to it by the Holy Ghost."
In 1649 an association was incorporated by Parliament, to be called "the President and Society for the Propagation of the Gospel in New England," whose object should be "to receive and dispose of moneys in such manner as shall best and principally conduce to the preaching and propagating the gospel among the natives, and for the maintenance of schools and nurseries of learning for the education of the children of the natives; for which purpose a general collection was appointed to be made in and through all the counties, cities, towns, and parishes of England and Wales, for a charitable contribution, to be as the foundation of so pious and great an undertaking." The society was revived by special charter in 1661, and Boyle was appointed president, an office he continued to hold until shortly before his death. The society afterwards enlarged its sphere of operations, and became the Society for the Propagation of the Gospel in Foreign Parts.
In the same year (1661) Boyle published "Some Considerations on the Usefulness of Experimental Natural Philosophy," etc., and in 1663 an extremely interesting paper on "Experiments and Considerations touching Colours." In the course of this paper he describes some very beautiful experiments with a tincture of Lignum nephriticum, wherein the dichroism of the extract is made apparent. Boyle found that by transmitted light it appeared of a bright golden colour, but when viewed from the side from which it was illuminated the light emitted was sky blue, and in some cases bright green. By arranging experiments so that some parts of the liquid were seen by the transmitted light and some by the scattered light, very beautiful effects were produced. Boyle endeavoured to learn something of the nature of colours by projecting spectra on differently coloured papers, and observing the appearance of the papers when illuminated by the several spectral rays. He also passed sunlight, concentrated by a lens, through plates of differently coloured glass superposed, allowing the light to fall on a white paper screen, and observing the tint of the light which passed through each combination. But the most interesting of these experiments was the actual mixture of light of different colours by forming two spectra, one by means of a fixed prism, the other by a prism held in the hand, and superposing the latter on the former so that different colours were made to coincide. This experiment was repeated in a modified form, nearly two hundred years later, by Helmholtz, who found that the mixture of blue and yellow lights produced pink. Unfortunately, Boyle's spectra were far from pure, for, the source of light being of considerable dimensions, the different colours overlapped one another, as in Newton's experiments, and in consequence some of his conclusions were inaccurate. Thus blue paper in the yellow part of the spectrum appeared to Boyle green instead of black, but this was due to the admixture of green light with the yellow. He concluded that bodies appear black because they damp the light so as to reflect very little to the eye, but that the surfaces of white bodies consist of innumerable little facets which reflect the light in all directions. In the same year he published some "Observations on a Diamond, which shines in the Dark;" and an extensive treatise on "Some Considerations touching the Style of the Holy Scriptures." Next year appeared several papers from his pen, the most important being "Occasional Reflections upon Several Subjects," the wide scope of which may be gathered from the title. His "New Experiments and Observations touching Cold" were printed in 1665. In this paper he discussed the cause of the force exerted by water in freezing, methods of measuring degrees of cold, the action of freezing-mixtures, and many other questions. He contended that cold was probably only privative, and not a positive existence.
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