original mixture are then perfectly separated, or at least as perfectly as this process can make them.

1280. We will treat the subject under the following heads-1st. That the process is one of great labour, if the mixture is a complex one. 2nd. Modifications of the process in special cases. 3rd. The process not an accurate one in all cases. 4th. Examples of the separation of

bodies by this process.

1st. If the mixture begins with a low boiling-point, and rises very high, the great labour required in such an investigation will be apparent, when we consider the number of fractions which will be obtained, even in the first distillation, and that each fraction, when submitted to the second fractionation, will probably be divided into four or five fractions. Thus, for instance, if the mixture commenced to boil at 200° F. and rose to 400° F., twenty fractions would be obtained in the first fractionation; and if the liquids were fractionated fifteen times, as was the case in the investigation of naphtha from Boghead coal, by Greville Williams, and in the investigation of the bases from Dippel's oil, by Dr. Anderson, 300 distillations would be required at least; but in each of the investigations we have named, 1,000 distillations were made.

We may here observe, that the boiling points of the fractions are lower in the second than in the first distillation, and this depression of the boiling point continues sometimes to take place up to the sixth fractionation, and the superior limit of temperature of the first distillation is extended in the second, &c. Example:-Mansfield, in his examination of light coal naphtha, observed a great depression in the point at which ebullition commenced in the case of each successive fraction in the second distillation; while, on the other hand, the thermometer index rose far higher than the superior limit of temperature, below which the same fluid was distilled in the first fractionation. For instance, the fraction which commenced from the boiling point, 230° F. in the first distillation, began to boil at about 196° F. in the second; while of the same portion, all of which was received below 239° F. in the first, more than a quarter remained unvolatilized at 248° F. in the second distillation, so that each fraction of 9° F. in the first was distributed over from 54° to 72° F. in the

second fractionation. This lowering of the boiling points is not due, in the majority of cases, to a decomposition of the substances, but to a gradually increasing separation of the less volatile portions of the mixture.

The student will see, from what has already been stated, that though a large retort will be required for distilling the original mixture, if complex, much smaller retorts will be required for distilling the fractions.

a

2nd. If it is intended merely to isolate a more volatile fluid from a less volatile one, Wurtz' apparatus (Fig. 24) may be employed with great advantage. The tube, a, is fixed by means of a perforated cork in the neck of the flask in which the liquid is to be distilled. The thermometer is fixed by means of a perforated cork in the opening, b, the bulb of the thermometer resting in the bulb, c. The tube, d, forms the exit tube. The portion of the vapour of the less volatile fluid which is carried upward by the vapour of the more volatile one, is condensed in the bulb, c, and flows back into the flask, whilst the vapour of the more volatile one passes on through the exit tube, d, and thus the two fluids become separated. Max Dürre found this apparatus very useful in the fractional distillation of the light coal naphtha obtained from the destructive distillation of brown coal.

FIG. 24.

Wurtz' apparatus is very similar in principle to the one which Mansfield contrived for the separation, on the large scale, of benzole from toluole, cumole, cymole, and other hydrocarbons in coal-tar. Mansfield's apparatus is shown in fig. 25. A is the retort, or boiler, which is charged with the coal naphtha, from which the vapours, as they are evolved, are conducted into B, the head, through the neck, C. D is a pipe which leads from the top of B to any convenient condenser, represented in the diagram by a still-worm, E; from the mouth of which, F, the benzole

water, which

need not be changed during the operation. His a secondary neck,through which the remainder of

the naphtha,

FIG. 25.

after the extrication of the benzole, may be distilled off, by turning on the tap, I. K is a pipe for running off the tarry residue.

As there is a wide interval between the boiling point of benzole and the boiling points of the other hydrocarbons we have named, the separation of the benzole from these other hydrocarbons becomes, on account of this difference, comparatively easy. When heat is applied to the retort, A, the temperature of the naphtha in A will gradually rise, until at last it boils; the vaporized portion, when it comes in contact with the cold inner surface of B, will be condensed, and flow back into A; B, and the water in G, become warmed by this condensation; the temperature of the water soon reaches nearly to the boiling point of benzole, which will therefore distil over rapidly, while so much of the toluole, and oils having higher boiling points, as is carried up into B with the benzole vapour, is still almost entirely condensed, and returned into the retort. The benzole continues to distil till the naphtha is exhausted of it, and the temperature of the water goes on rising until it boils; no liquid then remains in the retort that has not a boiling point at least 25° F. above that of water.

Sometimes it is necessary, in the separation of volatile substances, to make the liquid boil at a lower temperature

than it does at the ordinary pressure of the atmosphere; the boiling point may be lowered by distilling in an atmosphere of a gas which is lighter than that of atmospherie air; but the most effectual plan is to reduce the pressure of the atmosphere by connecting the apparatus with an air-pump.

When the difference in the boiling points of the liquids which have to be separated is less than from 40° to 50° C., it is difficult, if not impossible, to separate them by distil lation under the ordinary conditions: we are sometimes able to effect a separation in such cases, in distilling under a reduced pressure, because the difference in the tension of their vapours is frequently greater under the diminished pressure. This process is applicable, for example, to a mixture of alcohol and ether; less of the alcohol distils over when the mixture is distilled at a low temperature. These results are obtained if we effect the distillation in an apparatus in which a vacuum has been made, after the retort and the receiver have been placed in cooling mixtures of different temperatures, the receiver being placed in the one of the lowest temperature. We can place the retort, A (fig. 26), in ice, and the receiver, B, in a mixture of ice and crystallized chloride of calcium; the cork a, which attaches the retort and receiver, is covered with sealing

wax; also the cork b, which connects the receiver with the tube of lead: the other end of the lead tube is connected with an air-pump, C; the lead tube is provided with a stopcock, c. When the vacuum has been produced, ebullition commences; the stopcock, c, is then shut; the distillation continues, on account of the difference in temperature which exists between the receiver and the retort: for arresting the distillation the stopcock is opened, and air allowed to enter.

When the organic substances are oxidized by the air at the temperature at which they boil, they are distilled in a current of dry hydrogen or carbonic acid; to effect this, we fix in the tubulure of the retort a tube communicating with the apparatus in which the gas is disengaged, and it dips down to the bottom of the liquid to be distilled: before commencing this distillation, the entire apparatus is filled with the gas; the current of gas is continued during the whole time the distillation is carried on.

Instead of distilling alterable liquids in an atmosphere of an inert gas, we may distil them in a current of steam; the vapours of many oils have, for example, a tension sufficiently great, at the temperature of 100° C., for being carried over by the aqueous vapour, and condensed with it. Many of the essences to which plants owe their perfume can be isolated by this means.

When the liquid is a mixture of two bodies, we can sometimes succeed in separating them by heating the liquid with a chemical reagent which attacks one of the bodies without altering the other. This process can be applied to a great number of essential oils; these liquids are generally a mixture of a substance composed of carbon and hydrogen (frequently in the proportion of essence of turpentine), and of another substance, composed of carbon, hydrogen, and oxygen; caustic potash generally attacks the latter substance, whilst it is without action upon the former. In cases of this kind, we can either effect the separation by treating the essential oil with a concentrated solution of potash, which will dissolve the oxygen compound, or by distilling the essential oil off solid caustic potash, which will unite with the oxygen compound, and thus prevent it from distilling over with the other substance.

3rd. Even when a constant boiling point has been ob