318 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lodion of surgeons. This sac was tied with silk thread directly over 

 the end of the insulating plug. It was proved by trial that the 

 vaseline did not become so viscous under pressure as to refuse to trans- 

 mit the pressure with sufficient freedom, but the arrangement did not 

 prove itself as trustworthy as was to be desired. The collodion might 

 leak after several applications of pressure, which made it necessary 

 to reassemble the insulating plug and redetermine the elastic constants 

 of the apparatus, because the distortion included in the plug itself 

 was sufficient to introduce appreciable error. The device probably 

 could have been made usable with a little more effort, but it would 

 always have been more or less unsatisfactory, and would have been 

 applicable only to those liquids which do not attack the collodion, 

 whereas most of the organic liquids w^hich it was desired to use in the 

 future do so attack the collodion. The attempt to protect the coil 

 was abandoned after a month's work, therefore, and the method re- 

 placed by another, which at first sight introduced additional com- 

 plications, but which is really just as simple as the first, and has the 

 advantage of being applicable with only slight modifications to the 

 investigation of other liciuids. 



The modified method used two liquids in every determination, one 

 beside the one whose compressibility is to be measured. The water 

 under investigation is placed in a thin shell of steel fitting the inside 

 of the cylinder. This shell, when in position in the cylinder, is sur- 

 rounded on all sides and above and below by kerosene, wiiich below 

 transmits pressure to the manganin coil, and above reaches to the 

 moving piston with which pressure is produced. In the auxiliary 

 experiment to eliminate the effect of the distortion of the cylinder, the 

 shell w^ith water is replaced by a solid cylinder of steel, and the quan- 

 tity of kerosene remains the same as before. The motion of the 

 piston due to the change of volume of the kerosene remains the same 

 in the two experiments, therefore, and the difference of readings of the 

 two sets gives directly the difference of compressibility between the 

 water and the steel. The disadvantage of the method is that it is 

 not possible to use so large quantities of water as in the former method, 

 because the steel shell containing the water remains invariable in 

 length under pressure, and enough kerosene must be introduced origi- 

 nally to take up the change of volume of the water in this shell as well 

 as the distortion of the other parts of the apparatus. ^ The reduction 

 in the quantity of water under experiment is not greater than 30%, 

 however, and the other advantages more than outweigh this com- 

 paratively small loss of accuracy. 



