140 



Lord Rayleigh. 



[Mar. 23, 



found to be the case. After a little practice the manometer could be set 

 satisfactorily without too great a delay. When the pressure was nearly 

 sufficient, the regulating tap was closed, and equilibrium allowed to 

 establish, itself. If more gas was then required, the tap could be 

 opened momentarily. The later adjustments were effected by the appli- 

 cation of heat or cold to parts of the connecting tubes. At the close, 

 advantage was taken of the gradual rise in the temperature which 

 was usually met with. The pressure being just short of what was 

 required, and V being closed, it was only necessary to wait until the 

 point was reached. In no case was a reading considered satisfactory 

 when the pressure was changing at other than a very slow rate. It 

 is believed that the comparison between the state of things at the 

 top and at the bottom of the manometer could be effected with very 

 great accuracy, and this is all that the method requires. At the 

 moment when the pressure was judged to be right, the tap of the 

 globe was turned, and the temperature of the manometer was read. 

 The vacuum was then verified by the Toppler. 



The Weights. 



The object of the investigation being to ascertain the ratio of 

 densities of water and of certain gases under given conditions, the 

 absolute values of the weights employed is evidently a matter of 

 indifference. This is a point which I think it desirable to emphasise, 

 because v. Jolly, in his, in many respects, excellent work upon this 

 subject,* attributes a discrepancy between his final result for oxygen 

 and that of Regnault to a possible variation in the standard of 

 weight. On the same ground we may omit to allow for the buoyancy 

 of the weights as used in air, since only the variations of buoyancy, 

 due, for example, to changing barometer, could enter ; and these affect 

 the result so little that they may safely be neglected. "j* 



But, while the absolute values of the weights are of no consequence, 

 their relative values must be known with great precision. The in- 

 vestigation of these over the large range required (from a kilogramme 

 to a centigramme) is a laborious matter, but it presents nothing 

 special for remark. The weights quoted in this paper are, in all 

 cases, corrected, so as to give the results as they would have been 

 obtained from a perfectly adjusted system. 



* < Munich Acad. Trans.,' vol. 13, Part II, p. 49, 1880. 



f In v. Jolly's calculations the buoyancy of the weights seems to be allowed for 

 in dealing with the water, and neglected in dealing with the gases. If this be so, 

 the result would be affected with a slight error, which, however, far exceeds any 

 that could arise from neglecting buoyancy altogether. 



