26 On the Purification of Mercury by Distillation in vacuo. 



stopcock are fitted air-tight into the neck of the bottle*. 

 Thus fitted, the reservoir is placed on an ordinary adjustable 

 table-stand on the shelf H (fig. 1). To set the distiller in 

 action, open the stopcock, S, of the reservoir and pour some 

 of the mercury to be distilled through the thistle-funnel, t, 

 into the reservoir, and with a short piece of india-rubber 

 tubing and glass rod securely close the tube e (fig. 1) at the top 

 of the cistern. Then raise the reservoir. The mercury gra- 

 dually rises in the cistern, and by compressing the air in the 

 upper part is forced up the tube a b, and then filling the bulb 

 sprengels down the tube hi. The reservoir may then be 

 lowered on to its stand on H and the india-rubber stopper 

 removed from the tube e. The reservoir is set in action by 

 attaching a piece of india-rubber tube to the stopcock S and 

 sucking out air until, passing down the tube Z, it bubbles up 

 through the mercury in the reservoir. Then close the stopcock, 

 and adjust the reservoir at such a height on its stand that the 

 mercury is nearly at the topf of the bulb in the distiller. If 

 needful, a little more air is sucked out of the reservoir as 

 before described. Thus set in action, the level of the mercury 

 in the cistern c d will be retained constant until almost the 

 whole of the contents of the regulating reservoir have been 

 distilled. 



To start the distillation, remove the tin plate w T hich covers 

 the cylinder (H) and light the gas. Five to ten minutes later, 

 sufficient mercury will have distilled over to have entirely 

 displaced the impure mercury originally present in the narrow 

 Sprengel-tube i. 



The reservoir can be replenished with mercury without 

 interrupting the distillation. For this purpose it is only 

 necessary to place a screw pinchcock on the india-rubber tube 

 leading to the cistern of the distiller, open the stopcock S, and 

 pour the mercury into the reservoir through the funnel t. 

 Then suck a few bubbles of air out of the reservoir, as before 

 described, close the stopcock, and release the screw r -clamp from 

 the india-rubber tube. The level of the mercury in the dis- 

 tiller will remain as before. 



When it is desired to empty the distiller of mercury, air 



* So perfectly does this form of constant-level cistern work, that it 

 seems probable that it may prove useful for other purposes — e. g. keeping 

 a Sprengel-pump in uniform action for many consecutive hours &c. 



t The vapour-tension of the hot mercury will depress the level in the 

 bulb. The extent of this depression is somewhat dependent upon the 

 height of the gas. On this account a simple form of gas-pressure regu- 

 lator may be advantageously used with this apparatus. In another con- 

 nection I hope to describe a pressure regulator of very convenient construc- 

 tion for this and other purposes. 



