Page:The fairy tales of science.djvu/195

 Could we see through the sides of the kettle we should observe so many strange movements in the liquid that we might easily persuade ourselves that we were peering into some magic cauldron. By substituting a thin glass flask for the kettle, the whole process of boiling may be seen to perfection. On gradually heating water in such a vessel, we first observe the formation of tiny air-bubbles, which dart through the liquid with marvellous rapidity. As the temperature increases these "beaded bubbles winking at the brim" give place to much larger bubbles, which are formed at the bottom of the vessel, and which rise a little way in the liquid, and then contract and disappear in a most mysterious manner, producing a hissing or simmering sound. But as the heating goes on, these bubbles, which consist of steam, rise higher and higher in the liquid, till at last they reach the surface and escape, producing a bubbling agitation, or the phenomena of ebullition. It may now be remarked that steam itself is invisible, as the upper part of the flask appears quite empty; but when it escapes into the cold air it is condensed into a white cloud of minute drops of water.

It was first remarked by Gay-Lussac, an illustrious French chemist, that liquids are converted more easily into vapour when in contact with angular and uneven surfaces, than when the surfaces which they touch are smooth and polished. He also remarked that water boils at a temperature