394 Scientific Proceedings^ Royal Dublin Societij. 



tube contains the liquid under investigation, and is usually similar to the 

 simpler manometer. 



The iron apparatus is filled with pure mercury when the plunger is 

 drawn out as far as possible ; and the manometers, containing dry air freed 

 from carbon dioxide, are then inserted. The manometers are kept at a 

 known and nearly constant temperature by a current of cold water. 

 For a perfect gas the pressure is given by the formula 



T 



p = c . — 



V 



where c is a constant, f -^ ), jTis the absolute temperature, and v is the volume 



\ -'o / 

 of air in the manometer, ascertained by reading the position of the mercury. 

 In practice, the constant c for the low-pressure manometer was 

 ascertained by comparison with a direct column of mercury in a very long 

 tube placed temporarily in the third side-tube. This side-tube was then 

 closed by a glass rod ; and the constant for the high-pressure manometer was 

 ascertained by taking simultaneous readings on botli manometers. 



It is absolutely necessary that the liquid in the experimental tube should 

 be pure and free from air. The tube is filled in the following manner : — a 

 little pure dry mercury is first placed in the bulb-apparatus, fig. 4 ; and 

 while the pure liquid is being distilled — over phosphoric anhydride, if 

 possible — the distillate is collected directly in the bulb. 



The experimental tube is then placed in the bulb-apparatus in the manner 

 shown in the diagram. The bulb-apparatus and tube are exhausted by 

 means of a pump ; the india-rubber tube is then closed by means of the screw- 

 clip, and the apparatus is removed from the pump. 



By alternately warming and cooling the experimental tube some liquid 

 is caused to flow into it; and this liquid is vigorously boiled away. The 

 process is repeated several times so as to remove all air from the tube. 



