of H, F. RegnaulL 



277 



on the practicability of employing thermo-electric currents 

 as measures of temperature, and he concludes that in the 

 present state of our knowledge of their action, they can- 

 not advantageously be used for that purpose. 



The fifth memoir enumerates his experiments for de- 

 termining the absolute expansion of mercury. These arc 

 no less than 135 in number and show conclusively that 

 the coefficient of expansion of this body increases with 

 the temperature. Thus : 



Between 0° and 50° it is .00018027 



" 0° " 100° " " .00018153 



" 0° " 200° " " .00018405 



" 0° " 300° " " .00018658 



" 0° " 350° " " .00018784 



From these results he demonstrates that the expansion 

 of mercury may be represented by a parabolic formula of 

 two terms, thus : Dt = b T + c T.^ He determined the 

 values of the constants h and c to be as follows : 



h = .000179005 , c = .00000002523. 



The sixth memoir is on the law of compressibility of 

 elastic fluids. This law, it had hitherto been supposed, 

 was expressed with sufficient exactness by Mariotte^s pro- 

 position, that the product of the volume by the pressure 

 is a constant quantity. On air, Regnault made nine 

 series of experiments, comprising sixty-six experiments 

 in all, on nitrogen twelve series of ninety-eight experi- 

 ments, on carbon dioxide, ten series of eighty experiments, 

 and on hydrogen, eleven series of eighty-three experi- 

 ments. By a comparison of these data, he proves that 

 Mariotte's law is far from being rigorously exact. He 

 also points out a striking peculiarity exhibited by hydro- 

 gen when under pressure. While air and other gases 

 expand more than they should do, to be in accordance 



Trans, ix.] 36 



