198 Mr. J. E. Myers on a new Volumenometer* 



In making determinations with this instrument, an excess of 

 mercury is placed in the lower compartment of the vessel B. 

 The caps E and H are screwed down to fixed marks, and 

 pressure is then applied. Owing to the excess of mercury in B 



A B 



the volume-ratio ~- will be greater than the ratio ^r. In order 



to equalize these ratios mercury is run out by opening the 

 tap P. Repeated trials must be made by running out small 

 quantities, removing and then reapplying the pressure after 

 the vessels A and B have been put in communication with 

 each other and with the external atmosphere. The adjust- 

 ment is complete when the mercury stands at the same 

 horizontal level in both limbs of the manometer, after appli- 

 cation of the pressure employed. The instrument is now 

 ready for use. 



It is of importance that the pressures in the vessels A and 

 B shall be identical at the commencement of each experiment. 

 This equalization is effected by means of the tap G. The 

 body whose volume is required is placed inside the cavity, the 

 screw-cap replaced, and pressure applied to such an extent that 

 the compression is the same as in the preliminary adjustment, 

 as indicated by the level of mercury in the tube c. Owing to 

 the diminished volume due to the introduction of the body, a 

 further quantity of mercury must be run out in order that 

 the manometer may not indicate any difference of pressure. 

 The quantity which runs out is collected and carefully 

 weighed. Calculation shows that the product of the weight 

 multiplied by the constant of the instrument gives the volume 

 required. For let v represent the volume of B, and let the 

 total internal volume of B and D after the preliminary adjust- 

 ment has been made = (n + 1) v. If x is the volume of the 

 body introduced, then the volume originally (?i+l)y becomes 



after compression ^ — — - ~— — - . But if the volume of mercury 



withdrawn = dv, the same final volume is v — x + dv. Equating 

 these expressions, we obtain the simple relation 



x — dv=k.dv, 



n 



where h is a constant, provided the same compression is em- 

 ployed in all experiments. The constant k may be readily 

 determined by measuring with the instrument the volume of 

 a known weight of mercury. 



It is convenient therefore to start with the same initial 

 pressure in all experiments. The initial pressure employed is 

 that of the atmosphere, and in order to ensure that such may 



