Page:The fairy tales of science.djvu/201

 high mountain, we should find that the water would boil at a very low temperature, and never become hot enough to make a decent cup of tea. Thus at the town of Potosi, on the Andes, where the super-incumbent pressure of air will only support some eighteen inches of mercury, water boils at 188&deg;. Again, were we to carry our kettle to the bottom of a deep mine, we should have to heat the water to a point considerably higher than 212&deg; before it would boil, owing to the increased height of the column of air pressing upon its surface.

We now turn to the examination of another interesting point connected with the boiling of water. The reader will doubtless imagine that the hotter a vessel is into which water is poured the sooner the liquid will boil. This is far from being the case, as may be proved by pouring a small quantity of water into a silver basin heated to redness. Instead of flashing into steam, as might be expected, the water will gather itself into a globule and dance about on the hot surface as if bewitched. The liquid is in a state of incessant motion: sometimes it elongates itself into an oval in one direction; then, drawing itself up, it stretches out in a cross direction, and these changes take place so rapidly that a star-shaped figure or rosette is often the result. While the drop is in this spheroidal condition, as it has been called, let the lamp which heats it be withdrawn; the basin gradually cools, and after a short time the drop loses its spheroidal form,