96 CHEMICAL DISCOVEEY AND INVENTION 



of the pumps of this kind, one or other of which is to be found 

 in most laboratories. The Geryk (Fleuss) pump is a piston 

 pump in which the valves are immersed in oil. 



A very remarkable pistonless pump, introduced by Gaede, 

 has been described in Engineering for 20th September, 1913. 

 It is claimed for this " molecular " pump that a " vacuum " far 

 beyond that of any other pump is attainable. 



In ordinary air pumps a moving piston either solid or, in the 

 case of the mercury pumps liquid, divides the air in the space to 

 be exhausted into two parts, the residue being continually 

 reduced in pressure, but never entirely removed. In some cases 

 by doing away with valves between the space to be exhausted 

 and the pump cylinder, while the working parts in the latter are 

 immersed in oil, imperfections of workmanship are compensated 

 and the attenuated residue of air mechanically assisted to escape. 



The degree of exhaustion obtainable by some of these con- 

 trivances is roughly indicated by the following figures : 



Pressure reducible to 

 Pump used. mm. of mercury. 



Water injector 7-00 



Sprengel (mercury) .... 0-001 



Geryk (oil) ...... 0-000,2 



Topler (mercury) . . . . . 0-000,01 



Gaede (molecular) .... 0-000,000,2 



Charcoal in liquid air . . . . 0-000,000,8 



Electrolysis. In the chemical laboratory the electric current 

 is used in two directions, namely, in the production of heat as 

 already described, and for its application to electro-chemical 

 decomposition for analytical and other experimental purposes. 

 In the latter case in all ordinary operations a small current of 

 10 to 15 amperes is all that is necessary. The current is usually 

 obtained from a battery of accumulators of 12 to 24 cells, which 

 can be charged from the electric lighting circuit. Four cells will 

 usually furnish each working place, which is fitted with suitable 

 resistances, an ammeter for measuring the current, and a volt- 

 meter. 



In ordinary gravimetric analysis the weight of some precipitate 

 of known composition formed in the liquid by adding a suitable 

 reagent is the object aimed at. Take, for example, a solution 

 pf copper sulphate of which it is desired to know the amount, 



