24 



THE COMPRESSIBILITY OF LITHIUM, SODIUM, 



compressibility decidedly decreases under pressure, and in that of caesium 

 the decrease is very marked. The uncertainty of the pressure gauge and 

 the subtlety of the curves make the discrimination of the changes with 

 small differences of pressure somewhat doubtful, but a careful study of 

 the curves led to the following probable values : 



Change of compressibility zvith pressure. 



Thus, while sodium probably decreases in compressibility less than 2 

 per cent between 200 and 400 atmospheres (the mean pressure in each 

 case), potassium probably decreases over 4 per cent, and caesium over 20 

 per cent. Lithium and rubidium undoubtedly take corresponding places 

 in the table, although the present results with these substances were not 

 carried on upon a scale large enough to show this with certainty. 



The data given are enough to show that the five alkali metals are no 

 exceptions to the usual but not universal rule that the more compressible a 

 substance, the more its compressibility decreases with increasing pressure. 

 It is interesting to note also, that caesium, having a compressibility of about 

 the order of such liquids as bromine and chloroform, should have its com- 

 pressibility change with pressure to an extent not very different from 

 these liquids. 



The results recorded in this paper will be compared with those with 

 other substances and interpreted in the light of the theory of compressible 

 atoms in subsequent papers. 



It remains only to express the results of the work in terms of the three 

 standards of pressure commonly in use. The megabar is the pressure of a 

 megadyne per square centimeter, or 0.987 atmosphere. 



The average compressibility of the alkali metals 

 between 100 and 500 atmospheres. 



