But, as _caloric_ uniformly expands substances that are under its influence, one of the bodies most sensitive to _calorific_ effects has been selected to be the _indicator_ of the amount of _caloric_. This substance is _quicksilver_; and the scale of measurement, and the apparatus for exhibiting the rise or fall of the quicksilver, constitute the _thermometer_.
330. _If it is impossible to measure the amount of caloric in any substance, how can it be said that ice absorbs_ 140 _deg. in becoming water?_
Those figures simply record the amount of calorie indicated by the _thermometer_. The instrument will show with sufficient accuracy the _relative amount_ of caloric in various bodies, or in the same bodies _under different circumstances_, but it can never determine the _precise amount of caloric_ in any one body.
[Verse: "Great is the Lord, and greatly to be praised in the city of our God, in the mountain of his holiness."--PSALM XLVIII.]
331. _Why, if a hot and a cold body were placed near to each other, would the cold one become warmer, and the hot one cooler?_
Because _free caloric_ (that is, caloric that is not latent,) always exhibits a tendency to establish an _equilibrium_. If twenty bodies, of different temperatures, were placed in the same atmosphere, they would _all soon arrive at the same temperature_. The caloric would leave the bodies of those of the _highest_, and find its way to those of the _lowest_ temperature.
It travels in _parallel rays_ in all directions with a velocity approximating to that of light; and it passes through various bodies with a rapidity proportionate to their power of _conduction_.
333. _Why does melted metal run like a stream of fluid?_
Because _caloric_ has passed into its substance, and, repelling its particles, has separated them to that degree which produces fluidity.
334. _How do we know that it is caloric passing into the substance of the metal which produces this effect?_
Because, as soon as a bar of metal begins to be heated, it _expands_ and _lengthens_. It continues to do so, until the heat arrives at that point which _causes the metal to melt_.
335. _Why does the iron of an ironing-box sometimes become too large for the box to receive it?_