194 THE EARL OF BERKELEY, MR. E. G. J. HARTLEY AND DR. C. V. BURTON: 



it is judged that practically all dissolved air has been removed, tap H is closed 

 and G opened ; the aim is to sweep out any air adhering to the sides of A by a 

 current of moisture. Tap G is then closed, and the whole apparatus tilted to the left, 

 so that the liquid in B closes the left-hand orifice ; the tap H is then opened and F is 

 cautiously turned so as to admit air above the solution. In this manner the 

 compressibility tube is filled up to the tap without any air bubbles being apparent, as 

 a rule. The key of tap G must also be filled, otherwise deformation may take place 

 when the vessel is placed under compression. The compressibility tube is then taken 

 down and placed in the piezometer. 



Now, if the bulb B has been filled with a known volume of solution whose density 

 and concentration are known, and the change in weight in C is known, then the 

 concentration of the solution tilling the compressibility tube is known. 



All the experiments on compressibility here given were those obtained at C. 

 For this temperature, after the introduction of the compressibility tube, the whole 

 apparatus was filled with ice-cold water and the upper part, down to 3 or 4 cm. 

 below the coupling J of fig. 5, was surrounded by ice. A period of some four or five 

 hours, the ice being rammed down at intervals, was allowed to elapse so that a 

 constant temperature approximating to C. might be attained. The constancy of 

 the temperature was judged by readings of the mercury level in the compressibility 

 tube,* the readings being taken by means of a telescope. 



A pressure of some 20 atmospheres was then gradually put on the water in the 

 apparatus and when the level of the mercury in the compressibility tube was constant 

 it was noted. Another gradual increase of pressure of the same amount was then 

 put on and the level read, and so on till the maximum pressure of about 100 

 atmospheres was attained, after which the reverse process was carried out. 



The intention had been to reach a pressure of 150 atmospheres, but, although at 

 times the " liobax " tubing stood this, one or two serious breakages (causing in one 

 case the fracture of the compressibility tube) made it desirable to limit the experiment 

 to 100 atmospheres. 



Sources of Error. - 



There seem to be two main sources of error : 



(l) Variation of temperature during the experiment. There are two causes for a 

 change of temperature. One is that the ice in melting round the apparatus causes a 

 small quantity of warm air to come into contact with the gunmetal casting. This 

 was obviated by ramming the ice down at intervals of about 15 minutes. The 

 second cause of a change of temperature is unavoidable and is due to the development 

 of heat by the compression of the liquid itself. This thermodynamic change of 

 temperature was found to disappear in about 10 minutes. 



* In later work the piezometer was altered so that a mercury thermometer could be placed close to the 

 compressibility tube. 



