356 



ODOEOGEAPHIA. 



are conducted. On the large scale the vacuum is produced 

 by an air-pump worked by steam power.* 



"-^n^ 



The following are some of the forms of " aspirators," or 

 vacuum-pumps in use for small work. In these arrangements, 

 valves are dispensed with: — Fig. 6 is the "Sprengel": a glass 

 tube, somewhat longer than a barometer tube, dips into a 

 vessel containing mercury. An outlet near the top of thi& 

 tube is connected to the vessel to be exhausted by means 

 of a well-fitting piece of india-rubber tubing. The upper 

 end of the glass tube is connected to a funnel by tubing which 

 is provided with a clamp to enaljle it to be compressed if 



* Such stills are made in London hj J. Shears & Son and by A. Olilson. In 

 France they are made by Deroy, Paris- Grenelle ; Erojot, Paris ; and Berjot, 

 Caen. These names are mentioned for the information and convenience of 

 persons desirous of establishing; business of this nature. Vacuum stills con- 

 structed on this principle aie used for extracting the concrete " essences '" 

 mentioned in 1st Series, p. 6.3. 



