With such large apparatus several thermometers are required; one goes to the bottom of the still, thus permitting the temperature of the crude mixture to be ascertained, so that it is possible to recognize when the distillation is just beginning and when it is coming to an end. When the difference in temperature between the vapours which are distilling over and the residue in the still exceeds 500, the process should be stopped, or else the pitch will be charred.

The illustration on Plate X shows an installation for the distillation of β-naphthol; it is heated by means of three gas-rings, and is calculated for a charge of 1000 kilos.; such a distillation should occupy about 4 hours. The pitch constitutes about 5 % of the crude naphthol. The naphthol is allowed to solidify in moulds like sugar-loaves, and after disintegration it is "whizzed."

For liquids, worm-condensers may be used, or straight tubular condensers containing from 20-30 tubes. Condensers of this type may also be heated in order that diphenylamine or other easily fusible substances may be used with them if it is preferred not to distil them with steam (see p. 99).

Frequently also an observation window is placed in the vertical portion of the condenser, through which the stream of liquid may be accurately observed.

1 Naphthol pitch is an important commercial product. It is a black, brittle mass with a vitreous lustre, which is used as an insulating material for covering the joints of electric cables.

Fig. 27. - Diagrammatic arrangement for a laboratory vacuum distillation.

1. Screw clamp for capillary. 2. Capillary tube and Anschiitz flask. 3. Cooling water. 4. Receiver. 5. Manometer.

6. Safety tap. 7. Safety bottle. 8. To vacuum pump.

For laboratory use the arrangement shown in Fig. 27 works very well. The distilling flask possesses a double neck, one for the fine capillary and one for the thermometer, and at the same time this arrangement prevents the contents from spurting over. Heating should never be done directly, but always by means of an oil-bath, the temperature of which is 30-40o higher than the distilling temperature. The capillary is drawn out from an ordinary thin glass tube, and should be soft and flexible like a silk thread; it should just touch the bottom of the flask, and the upper part should be closed with a piece of pressure tubing and a screw pinch-cock. Just enough air should be allowed into the flask during the distillation to keep it going quietly.

An ordinary distilling flask with a long neck is used as a receiver, and not any complex piece of apparatus. If several different fractions are to be obtained, the distillation is stopped for a moment and the receiver changed, which occupies only a few seconds. The method of cooling may be seen from the sketch. The manometer is not placed in the pump circuit, but is attached to a separate tube in order to prevent any liquid from getting into it. The safety tap shown is of some importance, as it permits the introduction of a little air if the liquid in the distilling flask begins to show signs of frothing over; by this means any distillation may be carried out in a short time. The pump should be separated from the distilling apparatus by means of a large safety bottle. It is advisable that the water-pump be connected to the main supply pipe, in order to be independent of variations in pressure.

Method of carrying out the distillation. - The oil-bath is heated up to the correct temperature and the pump started. Usually a little water or solvent comes over first, so that the vacuum must be reduced by means of a safety tap. After a time the product becomes quietly fluid, and the capillary is regulated so as to allow a vigorous stream of very small air-bubbles to pass through. After a time the distillation begins, which offers no difficulties in the case of liquids, but the side tube tends to become stopped up in the case of solid products, such as β-naphthol, naphthylamine, etc. For this reason the neck of the flask is heated up before the beginning of the distillation, so that the first drops may be superheated. Sometimes it may even be necessary to heat up the portions where the corks are by means of a Bunsen flame, in order to force the distillate to pass over. With good quality resistance glass there is no danger of cracking, but at the same time it is always advisable to wear a pair of safety goggles. The distillation should always be carried out quickly; for example,

200 gms. β-naphthol can be distilled in 15-20 minutes, the receiver being kept moderately cool. The manometer is isolated from the rest of the apparatus, and is only connected up from time to time to test the vacuum. Beginners often daub their apparatus with paraffin, collodion, and other substances, in order to make them airtight, but this is quite unnecessary. The simplest method is to take good quality corks and to soak them well in hot, hard paraffin-wax beforehand, which renders any further treatment unnecessary. It is only necessary to use rubber stoppers when working with a very high vacuum, though for this purpose an apparatus that has been blown together is still better.

The distilled substance is melted by heating over a bare flame, and the liquid is poured into a small porcelain dish. The solidified product is pure and should not be further recrystallized.