SOLID ELEMENTS, AND THEIR PERIODIC RELATIONS. 67 



for compression always tends toward producing a smaller volume in a 

 system, while warming does not necessarily tend to produce a greater 

 volume. 



In every case it will be observed that volatility, which may be ascribed 

 to lack of cohesive tendency, seems to be associated with increased com- 

 pressibility, other things being nearly equal. This is seen most strikingly 

 on comparing the compressibility of chlorine, bromine, and iodine, but it 

 is manifest also in other cases ; for example, in the cases of arsenic and 

 .antimony, or sulphur and selenium. This effect of decreasing the cohe- 

 sive tendency is entirely in accord with principles which were laid down 

 in the recent communications from this laboratory concerning the signifi- 

 cance of changing atomic volume.* It is reasonable to suppose that sub- 

 stances already much compressed by their own great internal pressure 

 would not be sensitive to outside pressure. Further consideration of these 

 relationships will be taken up at a subsequent time in connection with the 

 compressibilities of simple compounds, for which a large amount of data 

 is at hand awaiting codification and discussion. 



In conclusion it is a pleasure to acknowledge the generous support of 

 the Carnegie Institution of Washington, which alone has made the present 

 investigation possible. 



SUMMARY. 



Attention is called to the summaries of the preceding papers, given on 

 pages 25, 40, and 52. 



In this last paper has been described the application of the method of 

 Richards and Stull to the determination of the compressibility of twenty- 

 one metallic elements. 



The compressibilities of these elements between 100 and 500 atmos- 

 pheres were found to be as follows, expressed in terms of megabars multi- 

 plied by 10 6 : Magnesium, 2.7; calcium, 5.5; zinc, 1.5; cadmium, 1.9; 

 copper, 0.54 ; silver, 0.84 ; gold, 0.47 ; palladium, 0.38 ; platinum, 0.21 ; 

 tin, 1.7; lead, 2.2; thallium, 2.6; aluminum, 1.30; iron, 0.40; manganese, 

 0.67 ; nickel, 0.27 ; chromium, 0.7 ; molybdenum, 0.26 ; arsenic, 4.3 ; anti- 

 mony, 2.2 ; bismuth, 2.8. 



This series of values depends entirely upon work at Harvard, except as 

 regards the standard of pressure. It is independent of other investigations 

 on compressibilities. 



It is shown by comparing these values with those of other elements pre- 

 viously found in the chemical laboratory of Harvard College, that com- 

 pressibility is a periodic property of the elements and that it is probably 

 in part associated with the same causes which determine atomic volume 

 and volatility. 



*Richards, Proc. Am. Acad. 



