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."