82 
CHEMISTRY: A.W.C. MENZIES 
Proc. N. A. S. 
useful in ebullioscopy, made from glass tubing a few mm. in bore and 
having, without its handle, a length of perhaps 12 cm. Permanent gas 
is removed prior to sealing by the process of boiling out familiar to many 
who have had occasion to measure vapor pressures. For reading the differ- 
ence of level of the two liquid surfaces, a mm. scale may be etched on both 
limbs. For many purposes, water is a suitable filling liquid; but it is 
obvious that the sensitiveness in any particular range of temperature may 
be given widely different values according to the rate of change of vapor 
pressure and the density of the liquid selected. The change of tempera- 
ture between upper and lower bulbs that will cause a change of level of, 
say, 1 mm. in the height of the column of the filling liquid may be com- 
puted in an obvious manner from the known vapor pressures and densi- 
ties of this liquid, and the results tabulated with temperature as argument. 
Thus, for water, at 57°, 80°, and 100°, these values are 0.01180°, 0.004969°, 
and 0.002599°, respectively. 
Although the thermometers are of different general types, as indicated 
above, it is perhaps of interest to compare this differential thermometer 
with the Cavendish- Waif erdin 2 metastatic type as elaborated by Beck- 
mann, 3 and as applied in the same field, for example that of ebullioscopy. 
With regard to length of scale per degree, the Beckmann mercurial type is 
limited by the usable size of bulb, and by the permissible narrowness of 
the capillary, so that a centrigrade degree corresponds customarily to 
40 or 50 mm. movement of the mercurial thread. The type here described 
is not thus limited in this respect, for a very low-boiling liquid may be used 
for filling. With the simple water filling, however, one degree centi- 
grade at 80°, a temperature close to the boiling-points of the two favorite 
solvents benzene and ethyl alcohol, corresponds to an observed change of 
length of over 200 mm. In both types the length of degree varies with 
the actual temperature. As to range, the Beckmann type is restricted 
to that between —39° and +250° C. 4 While this is a much larger range 
than can be conveniently covered by the use of a single chosen liquid in 
the newer type, the simplicity of construction makes possible such a wide 
choice of filling liquids that a much wider range is easily available in the 
direction of lower as well as of higher temperatures. In comparing pre- 
cision, one has to bear in mind, for the Beckmann type, possibilities of 
error due to (1) lack of uniformity of bore, (2) hysteresis in change of 
volume of bulb, (3) effect of pressure on volume of bulb, (4) sticking of 
mercury in capillary, (5) exposed thread, (6) difference of radiation to and 
from bulb, (7) departure of apparent degree from true degree. For the 
type here described, no one of the first six of these sources of error is im- 
portant, for reasons that will be sufficiently obvious. In regard to (6), 
it may perhaps be said that the change with environment of radiation loss 
suffered by a Beckmann thermometer at temperatures far from room tem- 
perature is here largely eliminated because suffered alike by upper and 
