OSMOTIC PRESSURES OF SOME CONCENTRATED AQUEOUS SOLUTIONS. 485 
A plunger A, working through a stuffing box in a cylinder, compresses a thick 
“ steam cylinder” oil. The pressure is communicated to another cylinder B by steel 
pressure tubing, and a steel plunger C works vertically in B and actuates the 
beam DE, which has a fulcrum at E. A pan F, suspended on knife edges, carries the 
weights, and a sensitive level is provided at G. The beam, on account of small 
leakages, does not remain horizontal, but has to he relevelled at intervals by means 
of the plunger A. 
A horizontal arm HI (see plan) is secured to the top of the plunger C (elevation) 
and is constrained to move to and fro by means of an oscillating system (not shown) 
worked from the laboratory shafting. Although the plunger works in its cylinder 
without any stuffing box, and the viscosity of the oil is the only preventive of 
large leakage, it was found advisable to move it continuously about its vertical 
axis—otherwise the pressure registered by the weights on the beam was not quite 
the same as the pressure delivered to the osmotic apparatus. 
Knowing the weights on the pan, the ratio of the arms of the beam and the 
diameter of the plunger, the pressure can be calculated—it was noticed, however, 
that, when big pressures were used, the calculated values did not quite agree with 
those shown by a standard pressure gauge—doubtless this was caused by a slight 
flexure in the beam. 
The cylinder B is in communication with a U tube L, and the pressure is delivered 
- on to the surface of the mercury—the mercury in the other limb of the U tube 
passes on the pressure to the solution in the osmotic apparatus. 
The Dead- Weight Standard Pressure Gauge. 
During the course of the experiments it was found that more concordant results 
could be obtained than had been anticipated. The beam apparatus already described 
only gave pressures agreeing among themselves to about I per cent. ; this was 
surmised to be due to the fact that the hardened steel balls at the fulcrum and at the 
top of the plunger gradually crushed the metal under them so as to form a 
depression, thus slightly altering the effective ratio of the arms of the beam every 
time it was put up. That some such effect was produced is shown by the fact that 
the oscillation of the arm HI (Diagram 2) caused a vertical oscillation m the beam, 
especially when the movable metal piece at the top of the plunger (not shown in the 
diagram) was worn into a depression. 
It was therefore decided to substitute one of Messrs. Schaffer and Budenberg’s 
dead-weight standard plunger gauges for the final determination of the equilibrium 
pressures; this apparatus was connected to the U tube at K, and a tap (not shown) 
was provided in the U tube head so that the beam apparatus could be cut ofl‘ 
when required. 
The dead-weight gauge, which is a commercial article, need only he described 
briefly ; it consists of two plungers, one supporting the weights, while the other, 
