335. _Why does the iron of an ironing-box sometimes become too large for the box to receive it?_ 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. [Verse: "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_. 338. _Is there any latent heat in air?_ Yes: a considerable amount. In a pint measure of air, though in no way evident to our perceptions, there lurks sufficient caloric to raise a piece of metal several inches square to glowing redness. 339. _How do we know that caloric exists in the air?_ It has been positively demonstrated by the invention of a small condensing syringe, by which, through the rapid compression of a small volume of air, a spark is emitted which ignites a piece of prepared tinder. 340. _What is the cause of the spark when a horse's shoe strikes against a stone?_ The _latent heat_ of the iron or the stone is set free by the _violent percussion_. The same effect takes place when _flint_ strikes against _steel_, as in the old method of obtaining a light with the aid of the tinder-box. [Verse: "The waters are laid as with a stone, and the face of the deep is frozen."--JOB XXXVIII.]