288 Dr. H. Herwig's Investigations on the Conformity 



stopcock at n the apparatus was thus kept unchanged for some 

 time. Hence the air collected itself over the mercury which 

 lay deep in the tube a b ; and when this had been effected, the 

 tightness of all the communications of the apparatus could be 

 simultaneously controlled. 



Then by varying the pressure of the air in the intermediate 

 part of the apparatus, the volume of the air confined in the tube 

 a b was made to vary, and that from the largest to the smallest 

 possible volume, while the simultaneous states of pressure and 

 volume were, naturally, measured with the bath at constant 

 temperature. To determine the pressure there were six mercury- 

 levels to be measured — besides those of the barometer and ma- 

 nometer, those in the tubes a b and c d. A very excellent ca- 

 thetometer with a corrected telescope, by Staudiger of Giessen, 

 which admitted of reading off to the tenth part of a millimetre, 

 was used for this purpose. From one set of determinations of 

 the simultaneous pressure and volume of the enclosed air, the 

 quantity itself was determined with perfect accuracy. They 

 could also be applied to the purposes of direct calculation in 

 afterwards measuring the total tension exerted in the tube a b ; 

 this, however, was never very great. 



Now the bulb filled with liquid was burst, and to obtain 

 the solution of the real problem, viz. the determination of the 

 volume, pressure, and density of vapour formed under differ- 

 ent circumstances, we proceeded as follows. The relation be- 

 tween pressure and volume, always at a constant temperature, 

 was to be sought from the point of saturation of the vapour up 

 to the point where, for this temperature, it follows Mariotte's law ; 

 and different temperatures would be investigated in this wise. For 

 this purpose, first of all, a definite temperature of the bath was 

 maintained with the greatest care, which could be effected very 

 readily by reason of the large size of the bath (25 litres) and 

 the mode of heating employed, which was scarcely disturbed by 

 draughts. It was possible to maintain the temperature invariable 

 within 0*1 of a degree for a series of hours, and during the time 

 of measurement to keep it steady to *05 of a degree. The 

 temperature being constant, then, as in the measurement of 

 the air, as large a volume as possible of vapour was produced, 

 and made to pass thence into a smaller volume by means of 

 the gradual introduction of air into the intermediate part of the 

 apparatus. However, before taking a measurement of the coex- 

 isting pressure and volume, a considerable pause was made each 

 time so as to allow the condition of the vapour to become sta- 

 tionary. The commencement of the stationary condition could. 

 be recognized by the repeated measurements. 



We may remark that the converse process (of passing to a 

 larger volume from the state of saturation of the vapour by 



