BY DR. LAUDER BRUNTON. 5G1 



to the bottom of the second. The first flask being then placed in a 

 beaker of warm water and the second in cold, the ether distils over 

 from the former into the latter and is condensed. When a large quan- 

 tity of the ether has passed over, the Masks arc transposed, when the 

 whole of the ether rushes back into the first flask. The process may be 

 repeated indefinitely. 



In connection with this subject, an arrangement may be described 

 which is chiefly used for washing precipitates. It is also applicable for 

 the purpose of replacing loss by evaporation when liquids are boiled, or 

 to keep the water at a constant level in a water-bath. (See fig. 331.) 

 It consists of a large flask, , fitted with a cork, through which pass 

 two tubes. One of these, &, c, is straight and open at both ends ; the 

 other, d, e, g, /, is bent so as to form a syphon, the limbs of which are 

 of equal length. Both ends of d, e, g, /, are at a somewhat lower level 

 than the lower end of Z>, c. The end is placed in the funnel or water- 

 bath at such a height that the level of the lower end of 6, e, coincides 

 with that at which it is desired that the surface of the fluid shall remain. 

 The effective difference in the limbs of the syphon is the space between 

 c and d. Whenever the surface of the liquid in the funnel or bath is on 

 a level with e, the tube d, e, g, /, ceases to act as a syphon ; but as soon 

 as it falls, d, /, again acts, and liquid runs into the funnel till the surface 

 is again level with c. 



208. Evaporation. Evaporation of watery liquids is usually con- 

 ducted in shallow basins of Berlin porcelain, heated either directly in a 

 sand-bath or over a water-bath. An ordinary saucepan answers per- 

 fectly as a water-bath. (See fig. 330.) If the naked flame is used, it 

 ought not to be allowed actually to touch the bottom of the basin. The 

 process is greatly accelerated by constant stirring. 



If it is important that none of the substance be lost, the liquid must 

 not be Jieated to boiling, as it is then apt to spirt over the sides. In 

 evaporating a solution to dryness, its surface often becomes covered 

 towards the end of the process with a pellicle, which hinders the vapor 

 below from escaping easily, and thus both retards evaporation and 

 causes the vapor to issue in jets which may occasion loss of material. 

 The formation of the pellicle is best prevented by stirring the fluid con- 

 stantly with a glass rod. It may also be prevented by covering the 

 evaporating basin loosely with another somewhat smaller one, or with 

 a concave glass with the concavity dowmvards, but this retards evapo- 

 ration. Solutions in alcohol, ether, and chloroform must be evaporated 

 in beakers. Solutions in the two latter menstrua must never be evapo- 

 rated over a naked flame, but always on a water-bath, as their vapor is 

 inflammable. 



Evaporation at a Constant Medium Temperature. It is sometimes 

 desirable to evaporate a liquid at a constant medium temperature, such 

 as 40 C. This may be done roughly by placing the evaporating basin 

 in a sand-bath, and carefully regulating the size of the flame by a 

 thermometer. Unless, however, it is constantly watched, the tempera- 

 ture is apt to rise or fall too much, and the solution may get spoiled. 

 This difficulty is avoided by using a water-bath heated by a gas-lamp, 

 which is connected with a Bunsen's gas-regulator. For this purpose I 

 find a water-bath of the accompanying form (fig. 331) a convenient 

 one. It is made of galvanized zinc, is eleven inches in diameter, and 

 five deep. At one side it bulges out, and in the projecting part thus 

 formed the thermometer and regulator are placed. The top of the bath 

 is covered by a zinc plate perforated by several large holes, in which 

 evaporating basins may be put ; or by a series of concentric copper 

 rings, one or more of which may be removed so as to accommodate 

 evaporating basins of different sizes. The regulator, as modified by 

 36 



