THE COMPRESSIBILITY OF LITHIUM, SODIUM, POTASSIUM, 



RUBIDIUM, AND C/ESIUM. 



INTRODUCTION. 



In a series of papers upon the significance of changing atomic volume,* 

 it has been shown that, at least in some cases, atomic volume is probably in 

 part dependent upon the intensity of the affinities concerned in holding 

 the material together. It has been shown, also, that even in those cases 

 where the relationship is not at first obvious, the same tendencies may still 

 be in action. In these obscure cases it seems probable that the difficulty of 

 interpretation is due to the supei position of conflicting effects produced by 

 the simultaneous action of chemical and cohesive affinity. 



Reference has been made to the increasingly definite hints of this rela- 

 tion, to be found in the literature of the subject. As far as has yet been 

 discovered, the first suggestion seems to have appeared nearly a hundred 

 years ago, when Sir Humphry Davy had a clear conception of the possi- 

 bility of this connection of effects, although he never amplified it. The 

 suggestion is contained in a footnote in one of his papers; it reads as 

 follows : 



The common metallic oxides are lighter than their bases, but potash and soda 

 are heavier; this fact may be explained on either theory; the density of a compound 

 will be proportional to the attraction of its parts. Platina, having a weak affinity 

 for oxygen, can not be supposed to condense it in the same degree as potassium 

 does ; or if platina and potassium be both compounds of hydrogen, the hydrogen 

 must be attracted in platina, with an energy infinitely greater than in potassium. 

 Sulphuric acid is lighter than sulphur; but phosphoric acid (where there is a strong 

 affinity) is heavier than phosphorus. The oxide of tin (wood tin) is very little 

 inferior to tin in specific gravity. In this instance the metallic base is comparatively 

 light, and the attraction for oxygen strong; and in a case when the metal is much 

 lighter and the attraction for oxygen stronger, it might be expected a priori that 

 the oxide would be heavier than the base.f 



It is interesting to note that this paragraph clearly gives to Davy the pri- 

 ority concerning this matter; but undoubtedly the later writers (including 

 the present author) developed the hypothesis independently of Davy and 

 of one another. 



*Richards, Proc. Am. Acad., 37, 1 (1901) ; 399 (1902) ; 38, 293 (1902) ; 39, 581 

 (1904). 



fHumphry Davy, Collected Works, 5, 133 (1840). We are indebted to Prof. W. 

 D. Bancroft for his kindness in calling attention to this note. 



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