The elasticity of air by the removal of the pressure gives rise to a variety of entertaining experiments. If a bladder containing a small portion of air be placed under the receiver of an air pump, while the air is exhausting the bladder will be observed to expand till it appears fully blown ; on the re entry of the air pressure will immediately reduce the included air to its primitive dimensions, and the sides of the bladder will collapse. At the larger end of an egg there is a bubble of air between the shell and the inner skin : if a bole be made at the smaller end, and the egg be placed with the hole downwards in a wineglass, under the receiver of an air pump, as soon as the air is begun to be withdrawn, the air within the egg will expand and force out the contents into the glass. When the air re-enters, by careful manage ment, the whole may be forced into the shell, so as to have its original appearance. Upon this principle fountains may be contrived. If a glass or other vessel similar to the one here represented, having a tube reaching nearly to the bottom, be half filled with water, and then placed under a tall receiver on the pump plate, the action of the pump commences, the air in the part a not being able to escape, expands itself as the external pressure is removed, and forces the water before it up the pipe, so as to form a continuous stream till the level of the water reaches the lower end of the tube. If the air in the part a could be compressed so that its elasticity might exceed that of ordinary atmospheric air, the fountain would act without being placed under a receiver. For this purpose
a condensing or compressing syringe would be necessary to force air into the upper part of the vessel. The compressing syringe differs but little from one of the barrels of the air pump. It has, however, no valve in the piston, but one at the end b, opening outwards, and which may be easily formed by tying over the hole a small piece of oiled silk. When this appa ratus is to be used, the end b is screwed into the mouth of the vessel into which air is to be forced ; and the piston being then raised above the hole a in the side, the syringe becomes filled with air : the piston is then depressed, and the air is, by its descent, forced into the vessel, and from which it cannot return on account of the valve at b opening only downwards. The piston is again raised till above the hole, and another barrel full of air is injected into the receiver. This process may be continued till the air is considered of sufficient density, which may be easily ascertained by knowing the proportionate capacity of the syringe and the receiver. If the receiver contain twelve times as much as the syringe, twelve changes of the syringe will be necessary to double the density of the air. It is, perhaps, scarcely necessary to remark that the receiver must be strong and furnished with a stop-cock or valve, so that when the syringe is separated from it the air may not escape. For fiarther informa tion on this science, See ATMOSPHERE, AIR, BAROMETER, &c.