62 MEASUREMENT OF HIGH TEMPERATURES. 
apparatus and the manipulations may be simplified. A form of vapor 
bath specially adapted to mercury calibrations is given in Fig. 3, one- 
tenth actual size. The boiler or retort A A A is of cast-iron, and is a 
modified form of the well-known mercury still of the shops. By drill- 
ing a hole through the bottom and tapping into it a long nipple, the 
central tube d d may be screwed down firinty in position. The nipple 
communicates below with a long tube of gas pipe, efg, leading to the 
condenser. The crucible A A A is closed above by a fiat lid, the edge 
of which as well as the rim of the crucible has been carefully turned, 
and is held in place by a stout gallows connection not shown in the 
figure, Through the center of the lid passes a smaller iron tube, h t, 
the lower end of which is closed and projects into the central tube d d, 
somewhat below the level a a of the surrounding mercury. It is into 
the tube h t that the hot junction of the thermoelement is to be intro- 
duced as far as the base of the tube. A lateral tube, b Jcl n, largely of 
iron, subserves the purpose of replacing the mercury lost by evapora- 
tion, and communicates with a larger reservoir, B B, in which the mer- 
cury is practically at the same level, m m, as in A A. Hence the level 
in B B is to some extent a gauge of the level in A A. A supply reser- 
voir, G (7, enables the operator to keep m m at constant height. To 
keep up the ebullition we made use of a kind of ring burner, consisting 
of three distinct blast-burners symmetrically placed around the crucible, 
the flames impinging upon its sides. In this way full advantage was 
taken of the current of air furnished by Professor Eichards's pneumatic 
pump. The operation of boiling may therefore be prolonged indefi- 
nitely. 
In the tables below we give a series of results by which the con- 
stancy of temperature attained during the successive stages of improve- 
ment of this apparatus is fully exhibited. These results will show that 
variations in the disposition of parts is by no means without conse- 
quence. 
Boiling point of zinc. — The construction of apparatus first used was 
carried out by Mr. Hallock, and to him the following description is due: 
In our earlier experiments an attempt was made to use the large clay 
retorts of the shops, but after some trials we abandoned them in favor 
of the special forms of retort now to be described. 
Having in mind the form aud operation of the ordinary apparatus for 
checking the boiling points of thermometers a retort was constructed 
with a view to surrounding the thermo-element with a double jacket of 
the vapor in question. It soon became evident that the simpler forms 
were useless, owing to the condensation of the vapors and the clogging 
of the outlets. We were thus led after trials of several other simpler 
forms, to test the arrangement shown in Fig. 4 and constructed as 
follows : 
(716) 
