Page:The fairy tales of science.djvu/190

156 represented on a small scale in a kettle of boiling water. “Mary, bring the kettle!” Heat, by entering bodies, expands them through a range which includes, as three successive stages, the forms of solid, liquid, and air, or gas; becoming thus in nature the grand antagonist and modifier of that attraction which holds corporeal particles together, and which, if acting alone, would reduce the whole material universe to one solid, lifeless mass. The influence of heat on the dimensions of material substances affords a convenient method of estimating the relative quantity of heat which will produce a given effect; for since it appears that a certain increase of temperature will invariably be accompanied by a certain degree of expansion of bulk, it follows that, if we can estimate the degree of expansion in any given case, we may thence infer the amount of temperature. Upon this principle depends the utility of those philosophical instruments called thermometers, or heat-measures. As we shall frequently have to refer to the indications of the thermometer, we will describe the construction of this beautiful little instrument.

The mercurial thermometer consists essentially of a fine glass tube with a bulb at one extremity, and which, having been filled with hot mercury or quicksilver, introduced through the open extremity, has been hermetically sealed while full, so that no