of vapour now issuing from the spout reminds us of the Arabian fable of the genie, who escaped from the fisherman's bottle in the form of a column of smoke. But the genie of the tea-kettle is infinitely more powerful than the genie of the bottle, who was, moreover, a stupid, blustering fellow, quite unlike our faithful servant, Steam. Let us see how our mighty genie may be evoked; in other words, let us ascertain the conditions under which vaporization takes place. Vapours, of which steam is the most familiar to us, are light, expansible, and generally invisible gases, resembling air completely in their mechanical properties while they exist, but subject to be condensed into liquids or solids by cold. Steam is perfectly invisible; but as soon as it comes into contact with the cold air, it is condensed into a white cloud, which consists of minute liquid particles.
When converted into steam, water undergoes a great expansion, a cubic inch becoming under ordinary circumstances a cubic foot of steam; or, to be exact, one cubic inch of water expands, when sufficiently heated, into 1694 cubic inches of steam. We have already shown that this change, like the liquefaction of solids, is effected by the addition of heat to the water. But a much larger quantity of heat enters into vapours than into liquids—into steam than into water. If over a steady fire a certain quantity of ice-cold water requires one hour to bring it to the boiling point, it will require a
