PHOSPHORUS, SULPHUR, AND SELENIUM. 



33 



before proceeding with the solid substances, so that this measurement 

 might be strictly comparable as regards the standard of pressure, and all 

 other conditions, with the measurements concerning the substances to be 

 studied. Obviously, it is not necessary to determine the compressibility 

 of water in each one of the jackets, for this quantity is one entirely inde- 

 pendent of the form or nature of the apparatus used. Nevertheless, expe- 

 riments were made with several of the jackets, in order to test their 

 efficiency. 



These compressibility determinations with water, as far as they concern 

 the present work, may be quickly given. In a previous research* two of 

 us found that water decreases 0.01632 of its uncompressed volume between 

 100 and 500 pressure-units. In three other experiments we have found 

 more recently the values 0.01664, 0.01652, 0.01644, and two others by Mr. 

 J. H. Mathews gave essentially similar values, 0.01640 and 0.01639. Clearly 

 the value is not far from 0.0164; and for most purposes this might be 

 taken as representing very nearly the average compressibility sought, 

 which is thus about 0.0000041. For our present purpose, however, the 

 greatest obtainable accuracy was desired ; and accordingly the following 

 three experiments were executed under conditions as nearly as possible 

 like those obtaining when the other substances were compressed. 



Compressibility of water (Jacket III). 



The careful plotting of these closely agreeing results leads to the con- 

 clusion that each gram of water requires 0.2031 gram of added mercury 

 between 100 and 500 pressure-units, after eliminating the compression of 

 the jacket. Dividing this quantity by 400 X 13.57, correcting for the vol- 

 ume of 1 gram, and adding the compressibility of mercury, the value 

 0.00004105 is found as the average compressibility of water, or 0.01642 

 as the total change in volume in water, between 100 and 500 pressure- 

 units, referred to the original uncompressed volume. This value, nearly the 

 mean of all the earlier work, will be taken in the work which follows as 

 being the one most nearly comparable to the other determinations. 



*Richards and Stull, loc. cit. See also Richards, Stull, and Bonnet, this mono- 

 graph, p. 14. 



