RECORDS OF MEETINGS. 383 



same defiuito fraction of the ()ri<!;inal volunio. This fact liccame 

 known as the " Law of Cliarles" or of (iay Lussac. Further study 

 of the subject was made by Dulong and Arago, in 1829, with the 

 design of ascertaining the hmits within whidi the so-cahed laws 

 were exact and later, by a great experimenter, Ilegnault, in 1847, 

 through whose ingenious processes highly important deviations 

 from the laws at high pressures (up to 27 atmospheres) were 

 found. Studies were made by him on the pressure of steam as 

 related to temperature and also measurements of the expansion 

 of mercury and some other liquids. Amagat in his researches in 

 1869 and the following years reached pressures up to 3,000 atmos- 

 pheres. The liquefaction and solidification of the so-called " per- 

 manent gases" by Cailletet and Pictet and the related studies by 

 Andrews of the "critical point" were also most important. 



Referring to liquids, the compressibility of water, that is, the 

 relation of ^•olume to pressure, was first measured by Jacob Perkins 

 of Xewburyport, a member of this Academy in 1820, and later 

 Oersted and others investigated the subject further. Amagat, 

 who worked also in this field, employed the highest pressures by 

 far that had been reached even up to 3,000 atmospheres, as already 

 remarked, and Tammann raised this to 3,500 atmospheres. But, 

 even before reaching these extreme pressures the leakage of the 

 packing and the joints of the piezometer became quite intolerable. 

 Also James Thomson and his brother ^Yilliam had shown in 1850 

 the effect of pressure in lowering the melting point of water. 



This rapid sketch will indicate sufficiently well for present 

 purposes what was the condition of this branch of physics at the 

 beginning of the work of ]Mr. Bridgman. 



About ten years since Mr. Bridgman began an extended series 

 of experiments upon the relations between the pressure, volume 

 and temperature which obtain with different substances, particu- 

 larly liquids and solids. Previous experimenters as just stattd 

 had found it impossible to maintain pressures greater than 3,000 

 atmospheres on account of the apparently insuperable difficulty 

 from leakage at the joints of the movable parts of the compression 

 apparatus. By a most ingenious device Mr. Bridgman succeeded 

 in overcoming this defect so completely that he was able to subject 

 the substance to be studied to pressures up to 20,000 atmospheres, 



