It is also believed that--all bodies are subject to the action of two opposing forces: one, the _mutual attraction_ of their _particles_; the other, the _repulsive force_ of _caloric_--and that bodies exist in the _æriform_, _fluid_, or _solid state_, _according to the predominance of either the one or the other of these opposing forces_. 329. _How do we measure the quantity of caloric in any substance?_ It is impossible to determine the amount of caloric which any body contains. Our _sensations_ would obviously be deceptive, since, if we dipped the right hand in snow, and held the left hand before the fire, and then immersed both hands in cold water, the water would feel _warm_ to the _right hand_ and _cold_ to the _left hand_. 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. 332. _How does caloric travel?_ 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_.