SECTION I

Of Heat and Light; and of the Modes of estimating their Degree.

Our sensations are a very imperfect measure of temperature; and when we wish to speak with precision on that subject, it becomes necessary to have recourse to other means of comparison. For the sake of the general reader, we shall, therefore, in the first place, briefly describe the principles of the construction of the Thermometer, the instrument for measuring heat.

All bodies, as we have shown in a former chapter, become more or less expanded, when they undergo an increase of temperature. Hence, the relative degrees of expansion of any body, may be viewed as a sort of measure of the degree of heat; and most of the thermometers employed, act upon this principle. Thus the common thermometer, as is well known, consists of a portion of some fluid, generally of mercury, enclosed in a small glass ball; the cavity of which ball communicates with a tube of narrow bore. We shall suppose the quantity of the mercury, and the size of the ball, to be so adjusted to each other; that when the instrument is placed in ice on the one hand, and in boiling water on the other; the whole expansion of the mercury between these two fixed temperatures, shall fall within the range of the tube.