312 PROCEEDINGS OF THE AMERICAN ACADEMY. 



secondly because in this and the following investigation a new method 

 for determining the compressibility was to be used, which had not 

 yet been proved to be reliable, but which could be tested by a com- 

 parison of the results obtained by this method with those already 

 obtained by another method at lower temperatures for water. 



In addition to the data for liquid water, two other quantities were 

 determined incidentally in the course of the work, and are given at 

 the end of the paper. One of these is the experimental measurement 

 of the compressibility and thermal dilatation of ice VI between 0° 

 and 20° and 6360 and 10,000 kgm. The other is the measurement of 

 the volume of kerosene up to 12,000 kgm. and between 20° and 80°. 



The Method. 



The method in its fundamental idea is as simple as it would well be 

 possible to devise. The substance, whose compressibility or thermal 

 dilatation is to be measured, is placed in a heavy steel cylinder in 

 which pressure is produced by the advance of a piston of known cross 

 section. The change of volume, given by the distance of advance 

 of the piston, is measured as a function of the pressure. The method 

 is simple, rapid, and above all, applicable to the highest pressures. 

 But there are a number of corrections which must be made, often 

 difficult to determine, which doubtless account for the slight use which 

 has been made hitherto of the method. Apparently, with the excep- 

 tion of the present work, it has been used recently only by Tammann,^ 

 and by Parsons and Cook.^ Tammann and Parsons and Cook 

 applied it only to the measurement of compressibility, reaching 

 pressures of about 4000 kgm. The author has previously applied 

 it to the measurement of the thermal dilatation of water at tempera- 

 tures below 0° C. over a pressure range of about 6500 kgm. 



The most serious of the errors which readily occur to one is that of 

 leak. It is almost essential to the success of the method to secure a 

 piston absolutely free from leak, and this has hitherto been a matter of 

 some difficulty at high pressures. Tammann did not entirely secure 

 this freedom from leak, but avoided it in large measure by the use of 

 a very heavy oil, such as castor oil, and still further lessened the error 

 by correcting for the slight amount of leak by nit^asuring the amount 

 of liquid which escaped past the piston in a given time. This method 

 would not be applicable to the highest pressures, however, because 



2 A. D. Cowper and G. Tammann, Z8. Phys. Chem., 68, 281-288 (1909). 



3 Parsons and Cook, Proc;. Roy. Soc. A, 85, 332-349 (1911). 



