POTASSIUM, RUBIDIUM, AND CESIUM. 9 



glass tubes or jackets were used to contain the mercury and the substance 

 to be compressed, according to the state of this substance. Also in the 

 course of the following investigations these forms were used, and some of 

 them were still further modified to meet the special conditions of individ- 

 ual cases. In general, it may be said that the method continued to give 

 entire satisfaction, provided only that air was scrupulously eliminated from 

 the substances to be studied. 



The present paper deals with the first part of this extended investiga- 

 tion, discussing the compressibility of the alkali metals. These were stud- 

 ied first because it was expected that they would prove to be very com- 

 pressible, and because they form well-defined simple compounds involving 

 great energy changes. In the following work other substances were 

 tested, so that in all the compressibilities of thirty-five elements and many 

 simple compounds were studied with sufficient care to leave no doubt as to 

 their relative values. Highly interesting relationships were found between 

 these values, showing not only that compressibility is closely related to 

 cohesive pressure as indicated by boiling-point and is, therefore, periodic 

 in the system of elements, but also other important connections between 

 the fundamental properties of material. In the following pages are dis- 

 cussed both the details of the experimental investigation and some of the 

 conclusions which may be drawn from the results. 



THE COMPRESSION APPARATUS. 



The Cailletet compression-pump used in the previous investigation served 

 well in this one also as a means of obtaining the high pressures needed. 

 The apparatus consists of a lever-and-screw compression-pump (A} filled 

 with oil, communicating through a stout copper tube with a heavy steel 

 barrel (5) provided with a stout steel screw cap (C~). The hydraulic 

 gauge (D} registering these pressures will be discussed under a subse- 

 quent heading. 



As before, the glass jacket () containing the mercury and other sub- 

 stance to be compressed was suspended under the oil and mercury in the 

 steel barrel, hanging from a bronze piece (F) held down by the screw cap. 

 The details of the arrangement can best be seen by inspection of the accom- 

 panying diagram (fig. 1). In order to keep the apparatus at constant 

 temperature, the barrel as far up as the screw cap was immersed in the 

 water of a thermostat, which varied less than 0.003 throughout the whole 

 time of the observations. This constancy was attained very simply by a 

 submerged reservoir of toluol (G), which forced mercury up and down in 

 a capillary tube, making contact with a very finely pointed platinum wire. 

 The weak current through this contact actuated a relay (H), which auto- 

 matically shut off the stronger heating current. The water in the ther- 

 mostat was forcibly stirred by means of motor-driven paddles (/). The 



