158 



In doing this, it is tacitly assumed that while the distilling is going on 

 the pressure in the still is exactly that shown by the manometer. But 

 this is only very rarely so; almost invariably the pressure in the distill- 

 ing vessel is a few millimeters higher, consequently the reading of the 

 pressure is almost always inaccurate and does not correspond with the 

 temperature indicated. The reason of this is that the efflux-tubes of 

 the distilling vessels are generally so narrow that the steam formed in 

 the process of distillation is unable to escape quickly enough, as a 

 result of which an excess of pressure is caused in the still. The 

 extent of this excess of pressure depends partly upon the force of the 

 distillation (i. e. the volume of steam which is generated within the unit 

 of time), and partly upon the diameter of the efflux-tube, and is com- 

 mensurate to the speed of the steam, i. e. to the rate of the volume 

 of steam which is passed through the efflux-tube per second in pro- 

 portion to its diameter. For the purpose of experimentally ascertaining 

 the excess of pressure, water was distilled from a copper cylindrical 

 still of about 25 liters capacity under reduced pressure. The still was 

 connected with a manometer and the receiver with another manometer, 

 so that the difference in the readings of the pressure indicated the 

 excess of pressure in the still. By inserting glass tubes into the 

 efflux-tube and by altering the force of distillation, the rate of velocity 

 of the steam in the efflux-tube and consequently the excess of pressure 

 in the still could be increased as desired. In the case of water, the 

 following figures were found, calculated from the known (variable) diameter 

 of the efflux-tube and the quantity of the distillate: — 



Rate of velocity of steam 1 m. = 0,5 mm. excess of pressure 



2 



» 



= 



I 



» >. 



6 



II 



= 



2 



« t 



12 



II 



= 



3 



» > 



22 



5> 



= 



4 



» >j 



34 



» 



= 



9 



II 13 



49 



>> 



= 



12 



yy i 



These relationships between excess of pressure and velocity of 

 steam enable us to calculate the extent of the former in a boiling 

 flask of from 50 to 100 cc. capacity, with a tube of 5 mm. width at 

 a distilling-speed of 0,015 g. per second: — 



Distillation 

 pressure 



Temperature 



Rate of veloc- 

 ity of steam 



Excess of 

 pressure 



Water. . . . 



n .... 

 Pinene . . . 



„ ... 

 Oleic acid . 



760 mm. 



50 » 



760 „ 



5 » 



4 11 



ioo u 



38,3° 

 I59i2° 



22,3° 



199° 



1,2 m. 



16,5 1, 



0,2 „ 



20,7 „ 



20 „ 



0,5 mm. 

 3 to 4 „ 

 o „ 



3 11 4 yy 

 3 11 4 yy 



