Beckek — Improved Methods of Evaporation in the Laboratory. 247 



the same time, the rotation of the bulb over the burner prevents any portion of 

 it getting superheated; and, in fact, the temperature of the liquid can be 

 regulated very exactly by the height of the flame used. With a plain bulb of 

 this tjT)e water may be rapidly boiled away with very slight formation of bubbles, 

 and consequently no loss by spirting. 



If a current of air be blowai through the bulb, a similar effect to that noted 

 in the experiments recorded in the first part of this paper is produced — namely, 

 that the liquid may be much more strongly heated without the formation of 

 bubbles. The air current may be produced in two ways : either by making use 

 of the rotation of the bulb to eject the mixture of air and vapour centrifugally, 

 or by blowing in air directly b^^, means of a filter-pump or bellows. Fig. 5, a 

 shows the modification designed to use the first method, consisting of the addition 

 of a wide bore T-pieee fused to one end of the bulb. When the bulb is revolved 

 at a moderate speed (about 150 r.p.m.), the mixture of air and vapour is thrown 

 out through the arms of the T by centrifugal force, and the air is consequently 

 drawn in through the other end. In this way the escaping vapour forms its 

 OAvn blower, and materially hastens the evaporation. Fig. 5, b shows the modifi- 

 cation suitable for use with an outside current of air. The air-jet is placed in 

 the bell-mouth of the tube, so that it draws in with the primary air a considerable 

 volume of secondary air, thus producing a current which completely removes the 

 vapour as it is formed. 



The latter method can be further amplified by supplying to the jet any 

 particular gas in which the liquid being evaporated may be most stable. Thiis 

 easily oxidisable substances can be evaporated in an atmosphere of coal-gas or 

 hydrogen, while readily hydrolysable chlorides may be evaporated in hydrogen 

 chloride. When the gas used is scarce or expensive, arrangements can be made 

 for circulating it continuously through the bulb, so that a limited volume would 

 suffice for a large volume of liquid. 



When the liquid has to be evaporated to dryness and the solid recovered, as 

 in the determiiiation of silica in analytical work or in the reerystallisation of 

 salts, a form of apparatus with a very wide opening at one end is used. This 

 is shown in fig. 5, c, and in its simplest form may consist of a round-bottom flask 

 of which the bottom has been softened and blown out, the neck cut off short, and 

 the whole mounted on a piece of glass tubing by means of a rubber stopper. 

 This allows the bulb to be revolved and a current of air blown through at the 

 same time, while the solid left behind can be scraped or washed out at the 

 conclusion of the evaporation. Incidentally it may be noted that, owing to the 

 action of the rotating b^^lb, solutions never become supersaturated, but the salt 

 crystallises out in very fine small crystals, even at the high temperature, as the 

 liquid becomes concentrated. In this way no difficulty is experienced in 

 ciystallising such difficult substances as potassium hydroxide and ferric 

 chloride. 



Owing to the complete absence of superheating, the apparatus is particularly 

 suited for use in evaporating at reduced pressure, as it obviates the violent 

 bumping which usually gives such trouble at low pressures. 



A form which has been found effective for this purpose is shown in fig. 5, d. 

 The revolving bulb is connected to the fixed end tubes by means of carefully 

 selected pieces of black rubber tubing, of such a size that when slightly moistened 

 they allow the bulb to revolve freely. When the whole is properly adjusted, 

 the power required to revolve the bulb is not large, even when the atmospheric 

 pressure is acting on the rubber joints, a small electric motor of 1/20 H.P. being 

 quite sufficient. 



