Even in cases where no regular Still has been on board, an extemporised Still has been made from a kettle, a gun barrel, or piece of lead piping, or anything of a similar nature.
The principle of the Still is simple enough, and is shown by the diagram, rather than drawing, on the right hand of the illustration. There is a vessel in which liquid is boiled. From the upper part of it rises a tube through which the steam must pass as it is generated. The tube in question is generally of considerable length, and is coiled inside a vessel filled with cold water, rendered colder by ice, if possible.
As the steam passes through the cold tube condensation takes place, and it becomes liquid again, but deprived of its heavier particles, so that if sea-water be placed in the still, the salt is left in the vessel, and nothing but pure water passes through the tube. In dissecting-rooms a small still is almost invariably kept. Many preparations are of such a nature that the spirit in which they are placed becomes discoloured, and has to be repeatedly changed. Now, even methylated spirit is an expensive article, and therefore, instead of being thrown away, the discoloured spirit is placed in the still, and reproduced in a clean and transparent state.
Nature affords innumerable examples of distillation, the chief of which are the Dewdrops which have already been mentioned. During the daytime the air is full of moisture drawn by the sunbeams from ocean. We cannot see it, but it is there, and when the chill of night cools the various trees, herbage, and other such objects, the aërial moisture is condensed upon them, which is then known by the name of Dew.
On the left hand of the illustration are shown the tiny Dewdrops as hanging on the slight threads of a spider’s web, and collected in larger drops upon a leaf.
There are many other familiar examples of the principle of condensation, the commonest of which is the so-called steam as it pours from the spout of a kettle. In point of fact, it is not steam at all, but only water condensed into very small drops. At the orifice of the kettle it is quite invisible, but when it passes into the air, and is condensed, the tiny globules become visible. The same fact may be noticed in the Napier’s Coffee Machine, which has already been mentioned. When the water is boiling in the glass globe no steam is visible, though the upper portion of the globe is entirely filled by it. But, no sooner is the cork removed, and the steam allowed to escape, than it at once becomes visible as a white cloud, being, indeed, a miniature copy of the rain-clouds that float above us.
Then there is that mostly invisible passage of liquid through the multitudinous pores of the body, which is generally known as perspiration. It is invisible in warm weather, but on a cold day is as visible as a rain cloud.
The Turkish Bath affords a good example of this fact. Sometimes the hottest room attains a temperature of 250° or more, water boiling at 212°. When a bather goes into that room, he appears to have a perfectly dry skin, the moisture being in the form of invisible steam, and swept off as soon as it is generated.
But, if he passes at once into the cold room, he is so enveloped in vapour that for a few moments he is wrapped in it as in a cloud, and can scarcely be seen, the vapour having been condensed by the cold air.