20 



NEW METHOD FOR DETERMINING COMPRESSIBILITY 



Therefore the progressive change in the value ft' ft" given above 

 must be due to the mercury alone, if the pressure readings were exact. 



The quantity ft", while constant for any one kind of glass varies 

 considerably with different kinds of glass. Amagat found values 

 ranging from 0.000002 2 to 0.0000025. 



In our case the constant ft" is easily found by subtracting the first 

 value of ft' ft" from the compressibility of mercury 3.80 x io -6 found 

 by Amagat. ft" = ft' (ft' ft") = (3.80 1.48) io~ 6 = 2.32 io~ 6 . 

 This result corresponds well with Amagat's. 



It is possible now to obtain a close approximation to the values of 

 ft' for pure mercury by adding the constant ft" to the values ft' ft". 



The values given in the last column of the table are computed in 

 this way. The}' are not certainly exact, but will amply serve the pur- 

 pose of this paper. 



Unfortunately Amagat does not state the temperature at which his 

 value for mercury 0.0000038 was found. Accordingly, by comparing 

 the behavior of a jacket filled with mercury at o with one at 20 , we 

 sought to trace the possible magnitude of the uncertainty. 



A change in pressure from 155 to 563 kilograms per square centi- 

 meter required an additional weight of mercury of 0.2486 gram or 

 0.01830 milliliter at o, while the same change in pressure required 

 0.2475 gram or 0.01828 milliliter at 20 . These two volumes are 

 almost exactly identical, hence ft' ft" does not change sensibly with 

 the temperature. In other words, the compressibility of glass and 

 mercury change by the same actual amount in 20 . But Amagat 

 found that the compressibility of glass changes about 3 per cent, for 

 ioo C. or about 0.000000014 for 20 . Hence if the compressi- 

 bility of mercury at 20 is 0.000003 So, the compressibility at o must 

 be 0.000003786. This difference of one third of one per cent, is too 

 small to produce any essential effect on our work, causing for example 

 only an uncertainty of one fiftieth of one per cent, in the compressi- 

 bility of bromine. 



