(27.) There are, however, various other means by which air may be partially expelled from a vessel besides the direct application of mechanical force. Thus if heat be applied to [Pg044] the vessel, the air, as has been already explained, will acquire increased elasticity, and will rush from the vessel with a force proportionate to the excess of its elasticity above that of the external air, and this process may be continued by increasing the heat to which the vessel is exposed, until a very considerable portion of the air has been expelled. If the orifice by which the air has escaped be then closed, and the vessel be allowed to cool, the air within, by having its temperature reduced to that of the external air, will lose all the elasticity which it had gained from the heat, and will be in the same condition as if an equivalent quantity of air had been withdrawn by any mechanical agent. The external air, therefore, will have a tendency to rush in with a force corresponding to the difference of pressures. The process of filling thermometers with mercury shows one use of producing a high degree of rarefaction by heat. To construct the instrument it is necessary to fill the bulb and a part of the tube with mercury; but the bore of the tube is so small that the mercury cannot be introduced by any ordinary means. It is therefore held over flame until heated to a high temperature. The air within it gradually increasing in pressure as its temperature is raised, is forced through the small bore of the tube, until the pressure of the air within becomes no more than equal to the pressure of the external atmosphere; this air being so rarefied that quantity in the bulb bears a very small proportion to its contents at common temperatures. The mouth of the tube is then plunged into mercury, and as the bulb cools, the air within it loses its elasticity, and the superior pressure upon the external surface forces the mercury into the tube. This continues until the air remaining within the bulb has been so contracted, that its pressure combined with the weight of the mercury, shall balance the atmospheric pressure. The tube is then reversed, and the air which remained rises in a bubble to the surface, and escapes.