(27.)
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.