1888.] Investigations on the Spectrum of Magnesium. 251 



has fallen below the end of the tube a, the gas in the tube to be ex- 

 hausted expands into A. The same process is then gone through 

 again and again, whereby the whole gaseous contents of A are each 

 time removed, and if the volume of A be large compared with that 

 of the tube to be exhausted, the pressure of the gas in the latter is 

 very quickly reduced. The bends bbb retain a little mercury when A 

 is exhausted, and prevent any diffusion from c into A, and from / into 

 c. Each time the mercury fills the apparatus a small quantity flows 

 over into C, but when it has risen above the opening of the tube 

 connecting C and B, it passes back into B, when the cock E is turned 

 so as to open the communication between C and B. 



VOL. XLIV. 



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