Critical Phenomena of Ether. 827 



The pressure is unaffected if 



y_ PS(c-a) / L ) 



c— 3a 



The movement of the float produced by the expansion of 

 the needle is negligible for moderate lengths of the latter, 

 since it depends upon the difference between the expansion 

 of glass and that of steel. 



Equation (I.) implies that if adjustment has been obtained 

 for a given pressure P, in order to obtain adjustment for a 

 pressure P+p, the increase v in the volume of mercury must 



be — - . If the two menisci move in cylindrical tubes 



c — da J 



of constant and equal cross-section, the point of the needle 

 must be raised through a distance 9(0 "~P) or _q 



( = px 0-0554). 



In practice the needle was soldered into a brass cap at the 

 height corresponding to P = 965 mm., which is the pressure 

 of aniline vapour in the neighbourhood of the critical tem- 

 perature of ether. 



L is a narrow graduated tube which forms a subsidiary 

 reservoir for fine adjustments of the mercury. If D is dis- 

 connected by closing slip M, small measured amounts of 

 mercury can be introduced into the gauge or withdrawn 

 from it. In this way it is possible to raise or lower the 

 temperature of the thermostat by successive steps of o, 01 or 

 less, the pressure being adjusted by the heating-coil. 



For considerable changes of pressure the reservoir D must 

 be used, and air must be pumped into or withdrawn from the 

 ballon through the three-way tap N. The heating-coil will 

 satisfactorily maintain constant pressure only if the tempera- 

 ture of the air in the ballon as indicated by thermometer P 

 is within certain limits (5° to 15° above the atmospheric 

 temperature) . 



The irregularity of boiling adds considerably to the oscil- 

 lation of pressure. In order to secure the best results it is 

 necessary to employ a gas supply of steady pressure and to 

 introduce broken porous ware into the aniline in the usual 

 manner. 



In a test lasting about three hours, during which the 

 pressure was observed for a few minutes at frequent intervals 

 by means of a water gauge, the extreme variation of pressure 

 observed was '29 mm. of mercury : this corresponds to a 

 variation in temperature at 193-4° C. of C, 12 0. If one 

 reading is omitted, the variation was 0'19 mm. or 0°*098 C. 



312 



