Vol. 7, 1921 
CHEMISTRY: MENZIES AND WRIGHT 
79 
barometric change, which gives molecular weights of solutes at their 
boiling-points by direct measurement of lowering of vapor pressure, without 
the use of any thermometer. Continuing this effort at simplification, 
he found that the application of the differential thermometer of the pre- 
ceding article to ebullioscopy with the Beckmann type of apparatus served 
only to render more glaring the inevitable irregularities inseparable from 
that method of procedure. When, however, Cottrell's paper appeared 
in 1919, the first opportunity was taken to secure collaboration in working 
out the simplest possible apparatus and technique which should free 
ebullioscopy from all its most serious drawbacks. 
The diagram, fig. 1, shows a boiling tube, whose narrowed upper por- 
tion itself serves as condenser tube. A glass cylinder, open above and 
below and of diameter slightly less than the boiling tube itself, is located 
concentrically within the latter, and confers the advantages of a double 
jacket. Within this is supported the differential thermometer, on the 
lower bulb of which hangs the pump, loosely, as a hat on a peg. 24 The 
absence of rigid connection diminishes breakage, besides adding flexibility, 
literal and metaphorical. The boiling tube is heated by a common Bun- 
sen burner, with a low flame protected by the customary metal draught- 
shield. When boiling begins, portions of the liquid in the bulb are carried 
by the vapor in fragments up the pump and discharged to form a thin 
film over the lower bulb of the thermometer, which thus attains the tem- 
perature at which this liquid, or solution, is in equilibrium with the vapor. 
The upper bulb acquires the temperature of the pure boiling solvent. The 
thermometer registers the difference between these two temperatures. 
Equilibrium is reached so rapidly that fresh additions of solute may be 
made as fast as they can be weighed out. Contrary to all ebullioscopic 
tradition, the apparatus will give good results in a room that is not free 
from draughts. Unlike every other ebullioscopic apparatus, this form is 
entirely free from all corks, ground glass joints or stoppers. 
The sources of error, especially that due to barometric change, eliminated 
by the use of the new type of differential thermometer have been briefly 
referred to in the preceding article. The advantages gained by removing 
the lower bulb from immersion in the boiling liquid have been well ex- 
plained by Cottrell 20 and confirmed by Washburn and Read. 25 The chief 
residual source of error lies in the uncertainty as to the true concentration 
of the boiling solution as deduced, not by subsequent analysis, but simply 
from the weights of solvent and solute used. Part of the solvent is absent 
from the solution in the form of condensate on the walls, and part is absent 
in the form of vapor. This source of error, ever present in the Beckmann 
type of apparatus, has often failed of due consideration. To minimize 
it, we constrict the boiling tube above the lower bulb, mark cc. gradua- 
tions on the cylindrical neck so developed, and read off the actual working 
volume of the solvent an instant after causing ebullition to cease, while 
