POTASSIUM, RUBIDIUM, AND C-TISIUM. 



The two hydraulic gauges used for the registration of the pressure were 

 made at different times by Schaeffer & Budenberg, of Zurich, Switzer- 

 land. One of these gauges, that used in the previous work, was labeled No. 

 2740747, while the other, procured a year later, was labeled No. 3014491. 

 As will be shown, the two gauges agreed very well with one another as far 

 as 500 atmospheres, and also agreed in general with those of the Brooklyn 

 establishment of the same firm, each gauge having been sent to Brooklyn 

 for verification at different times. Because the agreement of our own 

 gauges with one another was better than the agreements of the reports 

 received from the Brooklyn firm, and because also the mean reading of our 

 gauges gave a nearly linear curve for the compressibility of hardly com- 

 pressible solids with increasing pressure, while the Brooklyn result gave 

 curves less regular, the average reading of our two gauges was taken as 

 the standard, and every reading of each gauge was corrected to corre- 

 spond to it. 



In order to compare the two gauges together, they were both attached to 

 the same compression-pump on several occasions, and the pressure was 

 increased step by step, sometimes 50 atmospheres* at a time, and some- 

 times only 10 atmospheres at a time. The discrepancies between different 

 comparisons at the same point never exceeded an atmosphere, and the aver- 

 age was taken as representing the true relation of the gauges. The cor- 

 responding corrections are given in the following table, a positive correc- 

 tion signifying that the gauges in question gave too low a value for the 

 pressure. The gauges could scarcely be read more closely than to within 1 

 atmosphere ; accordingly, the corrections are not made more precise than 

 to within half an atmosphere. 



Corrections to be applied to gauges. 



[All the figures given below are kilograms per square centimeter.] 



LIBRA* 



Although the zero points of the gauges changed somewhat during the 

 protracted research, the relative corrections remained essentially un- 

 changed, and \vere applied with confidence to all the pressure readings 



*In this paragraph the phrase "atmospheres" is used to signify a kilogram per 

 square centimeter, in which terms the gauges were graduated. 



