Distillation

"B Y the action of heat liquids are vaporized, and the vapor in its turn becomes a liquid again on cooling. Suppose, now, there is a mixture of two liquids of which one turns to vapor more easily than the other. On the application of heat, with the exercise of a little care, the more easily vaporized of the two liquids will be the first to evaporate; and if this vapor, instead of being allowed to escape into the air, is held confined in a cool receptacle, it will return to the liquid state. In this manner the two mingled liquids will be separated, the one less easily vaporized remaining in the vessel used for heating them, the other being collected in another by itself. This operation of separating the two is called distillation.

"To fix this process well in mind, let us take an imaginary example. Let us suppose we have a quantity of water into which has been poured a considerable portion of ink. The liquid therefore is black, unfit for drinking and unfit for any other purpose requiring the use of water."

"But who would ever dream of drinking water as dark as shoe-blacking," Claire interposed, "or of using it to wash linen or cook vegetables?"

"Nevertheless let us see if there isn't some way to restore this water to its original purity, to separate it from the dark pigment in the ink and make it as clear and limpid as ever. Yes, there is a way: it is the method adopted in distillation. Water is easily vaporized, whereas the coloring matter in the ink is vaporized with extreme difficulty. If, then, we apply heat the water alone will rise in the form of vapor, while the dark matter will remain behind. Thus heat will bring about a separation that at first seemed impossible. All we have to do now is to confine the water-vapor and cool it until it returns to the liquid form; then the process is complete and we shall have in one vessel perfectly clear water, in another a turbid liquid containing the ink."

"If you had asked me," said Claire, instead of Clair], "to separate the two, the water and the ink, after they had once been mixed, I should have said it couldn't be done. And yet how easy it is! We heat the mixture and the separation takes place of itself. I should like to see this curious experiment."

"Nothing would be easier than to show it to you if we had the necessary apparatus. All that I can do at present is to show you a picture here that will help to make the process clear. We put the darkened water into a glass vessel called a retort, which expands at one end into a large globular flask, and at the other contracts into a long, tapering neck. The flask of the retort, when in action, is placed over a fire or flame."

Retort and Receiver

"Can glass be used for boiling water?" asked Jules.

"Certainly, if it is thin enough to expand uniformly when placed over the fire. The glass in this instance is of a quality that will bear heat if proper care is exercised in conducting the operation. Owing to its transparency it affords a clear view of what takes place inside, a circumstance of great importance when we desire to follow the successive steps of an experiment. The neck of the retort is inserted into another receptacle, likewise of glass and globular in shape, which is plunged into cold water. If heat is applied beneath the flask the water contained in it is vaporized, while the coloring matter is not. This vapor, as fast as it reaches the cooling receptacle, immersed as the latter is in cold water, loses its heat and returns to the liquid state. Thus we obtain perfectly clear water, free from all traces of ink. Spring-water is not clearer or purer; indeed, it is less pure, as you will presently perceive."

"That's all very clever and very interesting," observed Jules, "to be able to get clear water out of a bottle of ink; but what's the good of it? No one would ever think of such a thing as blackening water with ink just to turn it back into clear water by distilling it."

"Very true," was the reply. "I chose that example in order to make the process more striking to you. But if it is not our practice to obtain pure water for daily use by distilling it from a mixture of ink and water, it is no unusual thing to distil ordinary water, and for this reason: however clear and good to drink water in its natural state may be, it is never strictly pure. Whether it comes from a well, a spring, a river, or a lake, it has been in contact with the earth and consequently must contain, in however small a quantity, some of the soluble constituents of the soil. Would not water be salt if it ran over a bed of salt, and would it not be sweet if it ran over a bed of sugar? In like manner water that washes the soil is charged with the numberless soluble substances contained therein. Who has not noticed the earthy deposit left in course of time by even the best waters on the inside of bottles and pitchers and, in a still more marked degree, of water-pipes? What is this deposit except an incrustation gradually formed by the foreign substances dissolved in the water? No water, then, that comes in contact with the soil is pure, in the strict sense of that word. Rain-water, even when collected before it has reached the earth or washed the roofs of houses, is nevertheless impure; for it contains particles of dust swept down in its descent. I leave out of the account muddy water that owes its turbid condition to pelting rain or driving storm, also sea-water with its inevitable mixture of salt and its repulsiveness to the taste. Suffice it to say that all water in its natural state and containing however slight an admixture of foreign substances is unfit for certain manufacturing purposes, for example certain delicate operations in dyeing. Very often water may be most excellent for drinking, exactly suited to domestic uses, so irreproachably clear that the sharpest eye can detect in it no alien substance whatever; and yet for such purposes as those I have indicated it may be worthless.

"To give water the purity required in certain of the arts it is customary to distil it, not with an apparatus of glass, such as we use for a simple experiment like the one we have just been considering, but with a more substantial, more capacious outfit. The water to be purified is poured into a copper boiler, or alembic, or cucurbit, as it is variously called, which is sometimes provided with a hot-water jacket, and sometimes is placed directly over the fire. The steam ascends to a sort of dome, or head, surmounting the boiler, and thence by a long neck, called the rostrum or beak, it reaches a metal tube coiled in a spiral and hence known as the worm. This latter is immersed in cold water contained in what is called the refrigerator. In circulating through the worm the vapor becomes chilled and is condensed into water, which runs out at the lower and free end of the worm, the latter passing through the side of the refrigerator at its base.

Still

"It is plain that the water in the refrigerator must gradually become heated by the steam circulating in the worm, and thus be rendered unfit for condensing purposes. Hence it must be renewed from moment to moment, and this renewal is in fact made to go on continuously. Fresh water is run into the funnel shown in the picture as reaching to the bottom of the refrigerator, while the warm water, being lighter than the cold, rises to the top and runs out through the tube also shown. There is thus a constant renewal of cold water at the bottom of the refrigerator, with an equal outflow of warm water at the top. At the end of the operation there is found in the bottom of the boiler a muddy paste representing the impurities contained in the water subjected to distillation.

"Nothing is more disagreeable than to gulp down by accident a mouthful of sea-water. Nor is this water any better for washing linen than for drinking, since it will not dissolve soap; and it is equally unsuited for purposes of cooking. But by being distilled the water of the ocean, so unfit for our use in its natural state, becomes purified. Great ocean steamers are provided with distilling apparatus in which sea-water is freed from its salt, and the resulting liquid differs not a particle from that obtained by distilling fresh water. It is suitable for cooking and washing, but not the best kind of water to drink, because it holds in solution no air, a little of which is needed in all drinking-water. But it can be made to absorb the lacking ingredient by being shaken up in contact with the air."