Vacuum

A description of one of the earliest improved pumps is given in De V acuo Spatio. The cylinder is put vertically on a tripod, and the piston is pushed up and down from underneath by a lever and connecting rod. The top of the cylinder has two valves, a leather flap valve, and a plug valve. These valves are for alterna tive use ; when the air pressure is no longer sufficient the valve is withdrawn and replaced by hand at the correct part of the stroke. The spherical receiver is provided with a tap, which tap, as well as the connection between receiver and pump, is made air-tight by being covered with water held in the funnel-shaped bucket provided for the purpose. This is reminiscent of the mercury seals in use to-day.

It must be remembered that this pump was not constructed until after Boyle had published his account of the pump which we are about to describe, and that it embodied many features of Boyle's pump.

Robert Boyle and His Contemporaries.

Robert Boyle, the first of the great English school of vacuum enthusiasts, learnt of Guericke's early experiments from "a book that I had heard of, but not perus'd, publish'd by the industrious Jesuit Schottus," and set about constructing an air pump, in which task he had the help of Robert Hooke. The pump, which represented a great advance on Guericke's earlier efforts, is described in New Experiments Physico-Mechanicall, 166o. The piston was operated by a rack, and the operation of pumping was carried out with the help of a tap, controlling the passage to the receiver, and a hand operated valve consisting of a peg ground to fit into a hole bored in the top of the cylinder. The tap being open, the peg in place and the piston at its highest point, the piston was then fully withdrawn, and the tap closed. The valve was then opened by the withdrawal of the plug, and the piston raised, expelling the air. The valve was then closed, the tap opened, and the process repeated. The receiver was very large, and objects could be introduced through a lid at the top, cemented into place. W;th this pump Boyle carried out a variety of curious experiments, especially on the effect of the reduced pressure on animal life. About this time the members of the Accademia del Cimento were performing experiments with what may be termed the earliest mercury' pump, namely the Torricellian vacuum, which is pro duced by filling a long tube, closed at one end, with mercury and inverting the tube with its open end in a mercury cistern, when the mercury falls to the barometric height, leaving a vacuous space—a single stroke pump. The closed end of the tube was

enlarged, to enable objects to be placed in it. These experiments are described in the Saggi di Naturali Esperienze, published by the Academy in 1666. The fact that smoke falls in a vacuum was demonstrated with an apparatus of this kind.

Improvements in the piston pump were rapidly made. In 1669 Boyle published an account of his "second engine," in which he immersed the cylinder in water, and provided an automatic arrangement for opening and closing the tap. In this pump a plate was provided, to which the bell jar, used as a receiver, was cemented, an arrangement which is used till this day for certain experiments. About the same time Denis Papin worked with Huygens on the subject, and published in i674 an account of a pump provided with a plate for the receiver, and with a water gauge for indicating the pressure. In 1682 Boyle described a pump which Papin, who had worked as his assistant, made with him. It constituted a great advance in that it had two cylinders, the pistons being connected together by a cord passing over a pulley, so that the pressure which the outside air exerted on the one helped to raise the other—a great saving of labour. The pump also had automatic valves of leather. Another celebrated pump, made by Senguerd, employed a two-way tap, a device usually attributed to him, but actually employed earlier by Papin.

Hauksbee's Pump.

In 1709 Francis Hauksbee published in his Physico-Mechanical Experiments an account of an improved pump which long remained a pattern. It was a two-cylinder pump, the pistons being raised and lowered by a pinion wheel working between the racks attached to the pistons. The receiver was placed on a plate at the top of the pump, and provided with a mercury gauge disposed so as to be well protected from chance blows. The original pump is preserved in the collection of the Royal Society and may be compared with the picture published by Hauksbee in 1709. This type of pump, with certain improvements in the valves, persisted in use until the end of the nineteenth century. In some pumps made towards the end of the century special devices were introduced to diminish the disadvantageous action of the dead space still left when the piston has advanced to the end of the cylinder. In the older pumps this space fills with air at atmospheric pressure; the devices in question arrange that it shall fill with air at low pressure only.