J. H. Pratt, Jr. — Capillary Electrometer. 



143 



knows ; the deposition of the Triassic and Rhastic rocks under 

 conditions which no geologist has ever clearly pictured in im- 

 agination — at least to the satisfaction of his fellow geologists ; 

 the Triassic displacements and diking; the post-Triassic degra- 

 dation of thousands of feet of strata and the removal of the 

 debris to other regions — these and many other remarkable epi- 

 sodes have been completely blotted out of the geologic record 

 as commonly interpreted. But the Potomac formation" nar- 

 rows the hiatus : the formation itself carries the record back 

 from mid- Cretaceous time to the earliest dawn of the Cre- 

 taceous or the closing episodes of the Jurassic ; and the 

 post-Rhsetic and pre-Potomac degradation will tell the' story of 

 the Jurassic as eloquently, when men have come to read 

 geologic history in erosion as well as in deposition, as if the 

 deposits of the period were exposed to observation instead of 

 lying beneath the thousands of feet of newer strata forming 

 the Atlantic bottom. So while the hiatus is not yet closed it is 

 reduced by a fifth, a fourth, or perhaps a third of its length. 



Art. XII. — Experiments with the Capillary Electrometer of 

 Lippmann ;* by Julius Howard Pratt, Jr. 



The objects of this investigation have been, first, to determine 

 the limits within which the given form of electrometer can be 

 used ; secondly, to ascertain whether the polarization of the 

 mercury surface is such as to prevent the passage of a current 

 while the instrument is in use ; and, thirdly, to determine the 

 amount of charge retained at the polarization surfaces when the 

 mercury column is in equilibrium. 



The instrument used in these experiments was constructed 

 as follows : Y is a thistle tube, the tubing of which (A B) was 

 bent in the manner indicated (fig. 1) ; G is a small glass vessel 



* Abstract of a thesis presented for the degree of Doctor of Philosophy at Yale 

 University, June, 1887. 



