126 Henry Eiddell on 



The screws shown on the lower part were passed through 

 leather washers steeped in vacuo in melted lard, and they also 

 acted so perfectly that I have known the apparatus undisturbed 

 under ])ressure for weeks and at the end reading identically as 

 before the stop])agc, when reduced for temperature and barometer. 

 Of course the forcing in of the screws raised the pressure, two 

 being used to increase the range. A stream of purilied Carbonic 

 Acid was passed through the tube for some hours and the capillary 

 end sealed by heat while the lower *{3ortion was in the small 

 reservoir approaching the surface of the mercury. A stream of 

 the gas was continued past the open end for some time after 

 sealing and then the tube was passed beneath the surface of the 

 mercury, enclosing a measured volume of the gas, under barometer 

 pressure and temperature carefully recorded. The volume of the 

 tubes and the variations of internal diameter were very carefully 

 measured by weighing mercury contents and by the usual 

 calibrating method of passing a known quantity of mercury along 

 tho tube and noting the variation in the length occupied. The 

 result could be relied ujion to about one part in a million. The 

 volume of the gas was so calculated that the mercury began to 

 show in the capillary part of the tube at a pressure of about forty 

 atmospheres. Arrangements ai'e shown in the drawing by which 

 the required temperabure could be maintained surrounding the 

 tube with the Carbon Dioxide, while the air-tube, well screened 

 from the heat of its neighbour, could be kept at the temperature 

 chosen as the standard for the manometer. 



The results of the experiments are shown in the diagram 

 (fig. II), and can only be described briefly. 



It was found that, below a certain temperature, the carbonic 

 acid behaved as a partially saturated vapour, and that on adding 

 pressure the compression at first proceeded in a way somewhat 

 resembling that in a perfect gas, in which, so long as the tem- 

 perature remains unchanged, the product of pressure by volume 

 is constant, and a curve corresponding thereto would be an 

 hyperbola. In the figure the curves show the actual relations 



