OSMOTIC PRESSURES DERIVED FROM VAPOUR-PRESSURE MEASUREMENTS. 3 I 7 
If p x and r x be the values of p and r when the air enters the solution vessel, then 
the volume of air that passes through this vessel in time St is 
St, 
and the mass of vapour carried off is 
and the loss of weight will be 
Pl — TT* 
where t is the whole time of the experiment. 
In a similar way, if the dry air were passed through the solvent vessel the loss of 
weight would be 
l = * at. 
" 0 Pi) TTq 
But in the actual experiment the moist air passes from the solution to the solvent, 
so that the loss of weight in the solvent vessel is 
l 0 -l x = f ( Voff o _ difiA ) at. 
Jo \p 0 — 7T 0 Pi — 7rJ 
Since the mass of air passing through is the same at all points, r x p x — r 0 p 0 . Also 
as r is proportional to the effective driving pressure through the whole train of vessels 
(and to a factor depending on the dimensions of the apparatus), if we assume this 
pressure and the barometer to be constant, we have 
h _ Po _ P i ^ *-> ^ 
l x p x pu 7T 0 
but p x and p y are of the order of O'l mm. Hg below atmospheric pressure, whilst the 
difference between them is of order 0'004 mm. Hg ; and further, the barometric 
pressure, B, is of the order 760 and 7r 0 of 30 mm. mercury ; we may, therefore, as a 
first approximation write 
^0 Pu B 7T^ 
l x p x B 7Tq 
and assuming 
