On an Apparatus for Measurement of Low Pressures of Gas. Ill 



the mercury through a telescope and have the stopcock at the 

 same time at command. 



The volume-tube was calibrated in the usual way, by intro- 

 ducing weighed quantities of mercury into it, and making the 

 necessary corrections for the meniscus. The capacity of the 

 volume-tube, the globe, aud upper part of the tube e was 

 determined by inverting the apparatus and introducing mercury 

 through e until the mercury flowed down the pressure-tube; 

 the weight of this quantity of mercury, divided by the weight of 

 that contained in the volume-tube, gives the ratio between the 

 volumes; in the present case it is 1 to 54*495. While the appa- 

 ratus is being exhausted, the reservoir g is lowered so as to 

 prevent the mercury rising out of the tube /; but when it is 

 desired to make a measurement of the pressure, the reservoir is 

 raised and the mercury allowed to pass through the stopcock h. 

 On the mercury rising into the tube e it cuts off the communi- 

 cation between the gas in the globe and that in the rest of the 

 apparatus. Ultimately the whole of the gas in the globe is con- 

 densed into the volume-tube ; and its tension is then found by 

 measuring the difference of level between the columns of mercury 

 in the volume- and pressure-tubes. On dividing this difference 

 by the ratio between the capacities of the globe and volume-tube, 

 a number is obtained which is approximately the original pres- 

 sure of the gas ; this number must now be added to the differ- 

 ence between the columns, since it is obvious that the column 

 in the pressure-tube is depressed by the tension of the gas in 

 the remaining part of the apparatus ; on dividing this new num- 

 ber once more by the ratio between the volumes the exact 

 original tension is found. 



An example will best illustrate this. A quantity of gas was 

 compressed into the volume-tube, and the flow of mercury was 

 arrested when its surface reached the lowermost division on the 

 tube. The volume was then ^-A^— of its original volume, and 



5 4'4 9 5 P . ' 



the difference between the levels of the mercury in the volume- 

 and pressure-tubes was 66*9 millims. ; this number, divided by 

 54*495, gives 1*228 as the approximate pressure. 1*2 must 

 therefore be added to the observed column, which thus becomes 

 68*1 • and on dividing by 54*495, the number 1*2497 is ob- 

 tained as the actual pressure. 



The relations existing between the contents of the other divi- 

 sions of the volume-tube and the total contents of the globe 

 were determined by measuring the tensions of the same quantity 

 of gas when compressed into the different volumes. By this 

 means the values of the divisions 40, 35, 30, 25, 20, 15, 10, 9, 

 8, 7, 6, 5, 4, 3, and 2 have been found ; the experimenter is 

 thus enabled to employ a division suitable to the quantity of gas 



