Because caloric has passed into the substance of the iron, and repelled its particles, by which it has become expanded.
336. Why does the iron enter the box when it has become partially cooled?
Because a portion of the caloric has left the iron, the particles of which have drawn closer together, and contracted the mass.
"Cast thy burden upon the Lord, and he shall sustain thee; he shall never suffer the righteous to be moved."—Psalm lv.
This effect is frequently observed by females in domestic life, who, when they are ironing, or using the Italian irons, find that the heated metal has been too much expanded to enter the box or tube. They find it necessary to wait until the cooling of the iron has had the effect of reducing its dimensions. The expansion of bodies by heat is one of the grandest and most important laws of nature. We are indebted to it for some of the most beautiful, as well as the most awful, phenomena. And science has gained some of its mightiest conquests through its aid. Yet frequently, though quite unthought of, in the hands of the humble laundress, will be found a most striking illustration of this wonderful force of caloric.
337. Are there any instances in which the abstraction of latent heat will reduce the hulk of bodies?
Yes, there are several. But the most familiar one is that which is exhibited by mixing a pint of the oil of vitriol with a pint of water. A considerable amount of heat will be evolved; and it will be found that the two pints of fluid will not afterwards fill a quart measure.