394 
MR. W. CROOKES OK THE VISCOSITY 
and a pressure tube, c. The upper part of c communicates with the exhaust arm of 
the pump, and the tube e communicates with the upper mercury reservoir by means 
of a screw tap. The pressure and volume tubes are from the same piece, accurately of 
the same internal diameter, and are graduated in millimetres. The divisions in the 
pressure tube are numbered from below upwards, and extend to some height above 
the volume tube ; and the divisions of the volume tube are numbered from above 
downwards, the lowest division, 80, being on a level with the 0 of the pressure tube. 
The proportion between the contents of the volume tube and the globe was ascertained 
in the manner directed by Professor McLeod, and it was found that the globe, &c., 
from the point / had 111'8 times the capacity of the volume tube down to the 80th 
division. The action of the instrument is as follows :—Before the viscosity apparatus 
on the pump is exhausted the tap e is closed, in order to exclude mercury from the 
measuring instrument, the upper part being in communication with the pump and 
viscosity apparatus. The pump is now set to work, and the exhaustion in the globe 
keeps pace with that in the apparatus, there being free passage from one to the other. 
When the exhaustion is good it is advisable carefully to unscrew the tap e a little, to 
allow mercury to fill the two air-traps, cl cl, and rise in the tube as far as g. The tap 
e is then tightly closed, and the exhaustion continued. When it is desired to make a 
measurement of the pressure, the tap e is opened, and the mercury allowed slowly to 
rise. When it reaches the point f it cuts off communication between the globe a and 
the rest of the apparatus. The globe a now contains an accurately measured volume 
of highly rarefied gas of exactly the same exhaustion as that in the viscosity apparatus. 
The mercury continuing to rise, part ascends the pressure tube c, and part fills the 
globe, compressing the rarefied gas, until, when it reaches the lowest division, 80, of 
the volume tube, the gas is condensed to 111 ‘8 times its original volume. I now 
quote from Professor McLeod’s description :—“ Ultimately the whole of the gas in the 
globe is condensed 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 pressure 
of the gas : this number must now be added to the difference 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 number once 
more by the ratio between the volumes, the exact original tension is found. . . . The 
relations existing between the contents of the other divisions of the volume tube and 
the total contents of tire globe were determined by measuring the tensions of the 
same quantity of gas when compressed into the different volumes.” In this way a 
table has been constructed giving the value of each millimetre in the volume tube 
from 1 to 80. I generally make several readings at different heights of the volume 
tube, and take the mean. The McLeod gauge will not show the presence of mercury 
vapour. I have, however, entirely failed to detect the presence of mercury vapour at 
