POTASSIUM, RUBIDIUM, AND CAESIUM. 



11 



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50 and 100 atmospheres, and this first reading was taken as the starting 

 point of the determination. Minute air-bubbles were thus disarmed of 

 possible injurious effects. As already suggested, a weighed quantity of 

 mercury was now added through the open tube, and pressure was again 

 applied. The added pressure necessary to break the electrical circuit cor- 

 responded to the volume of the extra mercury introduced. This process 

 was repeated until the highest pressure was reached, and thus were found 

 the points on a curve which represented 

 the difference between the compressibil- 

 ities of mercury and glass. 



The mercury was now wholly removed, 

 except for a small amount in the U-tube 

 enough to seal the bend. The rest of 

 the left-hand side of the apparatus was 

 then filled with paraffin oil of known com- 

 pressibility, and the metal was introduced 

 in the form of a closely fitting cylinder. 

 The jacket was now placed once more in 

 the Cailletet barrel, and once more the 

 pressures corresponding to successive 

 added portions of mercury were found. 

 These new readings define the curve of 

 the total compression of the liquid, the 

 metal, and the residual mercury, all taken 

 together. By subtracting the correspond- 

 ing values from this first curve, and also 

 that corresponding to the known weight 

 of paraffin oil, results are obtained which 

 represent the differences between the com- 

 pression of the metal and a like bulk of 

 mercury. From this the compressibility 

 is very simply computed. 



In the case of rubidium and caesium an 



additional glass bulb was needed within the jacket. Correction for this 

 was easily made, as will be seen. 



If the platinum wire is very finely pointed, the fine tube around it about 

 1.5 mm. in diameter, and the mercury meniscus covered with perfectly 

 clean water, the indications of this instrument are surprisingly constant and 

 trustworthy. Even with a substance no more compressible than mercury 

 it is easy to be certain of the necessary pressure within 1 atmosphere a 

 very small fractional error in many hundred atmospheres. The pressure 

 at which the correction was made was taken as the true point, rather than 



V 



Fig. 2. Compression Jackets. 



