BARUS.] 
DEGREE OF CONSTANT TEMPERATURE. 
61 
register the height of the liquid. The capacity of the retort is about 
one gallon of liquid. If the liquid be fed in drops from a Mariotte flask, 
F, no interruption of ebullition need take place, 
In the apparatus given in the figure a space of constant temperature 
nearly 3 cm wide and fully 60 crn long is available. If the ring burner 
be so adjusted that the ebullition is fairly brisk the temperature within 
this space is almost perfectly constant. For relatively high temperatures 
it is generally advisable to discard the glass tube Jc m, and to close the 
neck with a plate of metal suitably perforated to allow the introduction 
of thermo-couples. In general, however, the large tubular space is con- 
venient both for the comparison of long-stemmed mercury thermometers 
and for the further comparison of electrical pyrometers with them. 
We omit special data relative to the degree of constant temperature 
here obtained, because it is questionable whether at these relatively 
low temperatures the use of large forms of vapor baths is to be recom- 
mended, and because none of the data of importance below depend 
upon the perfection of the large retort here described. In Chapter II 
data for smaller forms are fully given, and from these the efficiency of 
the larger forms may easily be inferred. 
Fig. 3. Boiling-point apparatus for mercury. Scale, T V 
Apparatus for mercury. — The boiling-point apparatus just described 
is useful for substances boiling at a lower temperature than mercury 
when constancy of boiling point can not be accurately relied upon. 
Such substances are mostly of organic kind, and are apt to change 
their properties slightly after long ebullition. It is for this reason that 
the mercury thermometer remains indispensable. When, however, the 
boiling point is fixed and known, like that of mercury or sulphur, the 
(715) 
