l6 NEW METHOD FOR DETERMINING COMPRESSIBILITY 



first or mercury series by dividing the added weight of mercury by the 

 product of the total weight of mercury and the added pressure. 



P'-P" = Wrp- ( 2 > 



For the absolute value of ft' ; we have had to depend upon Amagat's 

 work, as already stated an investigation that gives this value only as 

 far as fifty atmospheres. Since the magnitude of ft' is never greater 

 than 0.00000380, a slight variation in it with the pressure can be of 

 no serious importance in the present work. In order, however, to 

 make as easy as possible the correction of our results in case our as- 

 sumed value of ft' is in error, there is recorded below not only the 

 value ft, but also the value ft ft'. _ This quantity ft ft' relies essen- 

 tially upon our own work, and at any time the compressibility ft may 

 be found from it by adding the most probable compressibility of 

 mercury. 



The same apparatus serves for determining the compressibility of 

 solids ; but with liquids which do not attack mercury a still simpler 

 device may be used. The thin walled bulb may be dispensed with, 

 and the jacket itself used to contain the liquid. In this case a doubly 

 bent tube must be attached above, in order to contain the mercury 

 necessary for making electrical contact. The apparatus thus assumes 

 the form shown in Fig. 3, the stopcocks being affixed to facilitate fill- 

 ing. For the most accurate work it would be better to omit these 

 stopcocks, and to fill the jacket by exhausting the air ; because the 

 stopcocks are liable to leak unless very well ground, and their presence 

 introduces a slight uncertainty due to the small amount of liquid con- 

 tained in their channels. In our experiments this small volume, 

 amounting to only 0.002 of the whole, could be safely neglected. 

 It is well not to heat the glass to a high temperature, during the 

 filling, because of its well-known volume lag. On the other hand, 

 we have as yet been unable to detect any appreciable volume lag 

 on compression. This is shown by the fact that series of experiments 

 made by taking out mercury after the attainment of high pressure give 

 results identical with those obtained by gradually adding mercury. 



After being thoroughly cleaned this jacket was filled with mercury 

 and the stopcocks were closed. The mercury was arranged at a level 

 slightly above the lower point of the upper wire and the funnel tube 

 above was filled with pure water as before. 



The jacket was placed in the Cailletet barrel, and the pressures 

 corresponding to successive added portions of mercury were found in 

 the way already described. Thus the mercury curve was determined. 



