1916-17.] The Adsorption of Sulphur Dioxide by Charcoal. 165 
the mercury in the manometer was lowered, the requisite taps opened, and 
the freezing mixture removed until the manometer indicated the approxi- 
mate pressure required. The taps were then closed, the freezing mixture 
replaced, and the mercury adjusted to touch the blue tip again. Mercury 
was also run through the tap behind the manometer before it was shut. 
The tap between the manometer tubes was now closed and that above the 
charcbal bulb opened, and the adsorption proceeded at constant volume. 
At the close the mercury was first roughly adjusted, and then on opening 
the closed manometer tap the pressure was accurately found with the 
mercury touching the fine blue point. 
The heat of the adsorption vaporised some of the liquid dioxide around 
the charcoal bulb. This gas passed through the mercury bubbler and 
caused an increase in pressure, indicated on the oil gauge. Mercury was 
run out of the reservoir to keep the pressure constant. The volume 
thus collected every ten minutes was found by weighing to the nearest 
gram. A small correction was made for the volume of liquid dioxide 
and the thistle funnel tube ; and another when necessary was applied 
for change in temperature of the initial volume of gas in the aspirator. 
The security of the joins, corks, etc., in the collection apparatus was 
regularly tested by pouring in mercury through the thistle funnel and 
finding if the amount recovered on adjusting the pressure was the same 
plus the natural leak during time of adjustment. When the gas was not 
being collected it passed through the tap shown above the reservoir into 
absorption vessels. 
A short initial run was made, but owing to the fact that the liquid in 
the calorimeter had fallen below the shoulder of the charcoal bulb the 
heat effect registered was much less than it should have been. Two other 
complete runs were made, taking the pressure of adsorbed gas up to atmo- 
spheric. The comparison of the two sets of readings gives some idea of the 
accuracy of the calorimeter and method employed. 
It is now necessary to indicate what calorimetric quantity is measured — 
that is, what is meant in this case by the “heat of adsorption.” There 
may be defined, following Donnan,* three isothermal heats of adsorption, 
viz. : (1) equilibrium — the adsorption proceeds so that the vapour phase is 
constantly in equilibrium with the adsorbed phase ; (2) at constant pressure 
— the gas at constant pressure, but not necessarily at equilibrium pressure 
is picked up by the adsorbent ; (3) at constant volume — the total volume 
of the system is constant, and the gas is adsorbed with a fall of pressure. 
The heat effect registered by the author’s arrangement was the isothermal 
* The treatment by Freundlich, Kapillarchemie , pp. 107-11, is confused and erroneous. 
