280 E. W. Mbrley — Volwtnetric Composition of Water. 



verified as often as desired, and did not change during the 

 series of experiments. 



The measurement of the reduced volume of a gas is effected 

 by adjusting the apparent volume till the mercury in the 

 pressure tube stands at one of the Jolly points, and measuring 

 the level of the mercury in the eudiometer. This reading 

 determines the apparent volume, and the difference between 

 this reading and the reading of the zero point gives the 

 pressure under which the gas is measured. A convenient 

 interpolation formula reduces the computation to the addition 

 of a constant to the reading, taking the doubled logarithm of 

 the sum, and adding a logarithmic constant. 



The measurement of the level of the mercury in the eudi- 

 ometer is made by means of a reading microscope. This is 

 carried on a cylinder supported on the frame work of the 

 instrument, and provided with a tangent motion. On this 

 cylinder slides a piece which can be tightly clamped, and 

 which carries ways on which the microscope can be moved 

 vertically by a micrometer screw. When the microscope is 

 made to give distinct vision of a scale engraved on the 

 eudiometer, the terminal lines of an eye-piece micrometer are 

 made to agree with two successive millimeter divisions of the 

 scale. As these are but the three hundredth of a millimeter 

 wide, the coincidence can be made accurately. When coinci- 

 dence is secured, the microscope is made to give distinct vision 

 of the meniscus and the illumination arranged. When the 

 mercury is now made to coincide accurately with one of the 

 Jolly points in the pressure tube, the reading of the fraction of 

 a millimeter by which the mercury in the eudiometer stands 

 above a millimeter division of the scale is accomplished in a 

 few seconds. Great care was taken in the calibration of the 

 eudiometer. For this I fused to the end of the recnrved 

 tube, mentioned before, a further tube opening downwards 

 Through this air-free water was introduced into the eudi- 

 ometer. The mercury reservoir being raised, mercury was 

 admitted by opening the iron stopcock against the proper stop. 

 Water then began to drop from the added tube. The rate of 

 admission of mercury must be so slow that the amount of 

 water left adhering to the inside of the eudiometer is small, 

 and is nearly constant from one experiment to the next. 

 Three hours for the admission of eighty centimeters of 

 mercury gave concordant results. When the mercury reached 

 the lowest point to be calibrated, the iron stopcock was closed, 

 and the level of the mercury and the temperature of the water 

 determined. The tube from which the water had been drop- 

 ping was wiped in a constant manner, and a tube put in place 

 to collect the water which now issued on opening the iron 



