86 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
extending quite through the tube a a a a and open at both ends. The 
tube d d is wide enough to admit a mercury thermometer at its upper 
end, held in place by a cork, g, secured by an end of rubber tubing. 
The lateral tube of the upper cork ee is here somewhat wide and long, ' 
its object being to allow of the escape of vapor, should this be neces- 
sary. By suitably regulating the flame of the ring burner, however, 
the heat applied may easily be made such that the vapor condenses 
near the middle of the tube fg and returns to the liquid below. Thus 
the process is again continuous. A sharp line of demarcation around 
the tube fg shows the part of it where the temperature is just low 
enough for the condensation of the liquid used. 
Many experiments were made with each of these three forms of ap- 
paratus, the results of which will be more appropriately given below in 
connection with other similar experiments. 
Perfected form of boiling tube. — The three forms of apparatus for low 
temperature just described, each of which contains certain special de- 
siderata, can be combined into a single form adapted to the divers cases 
specified. This final form is given in Fig. 10, scale J. The glass parts were 
made for me by Messrs. Whitall, Tatum & Co., in Philadelphia. Special 
care is of course to be taken to have the tubes well annealed. As the 
lettering of Fig. 10 is identical with that of Figs. 7 to 9, only a few addi- 
tional words of exx^lanation are necessary. The tube a a a a is completely 
closed above by the cork ee, and communication with the atmosphere is 
effected by the bent glass tube h ?", ending in a little vessel, g, open below 
and rilled with asbestos wool. The tube g is a filter, catching noxious 
mercurial fumes should any such escape. It also impedes entrance of 
air when the boiling tube is filled with some other gas. The central 
tube d d is closed, of course, either with a perforated cork carrying a 
mercury thermometer, or at higher temperatures with asbestos wicking 
pushed downward into the tube almost as far as the point o of the thermo- 
couple. The position of the thermo-couple o a and o ft is pretty well 
represented, the two wires being held apart by a doubly perforated in- 
sulator, o y, of porcelain or of fire-clay. The clamp attached to the lower 
end of the tube d, which holds the insulator o y in position, is easily im- 
agined, and is therefore omitted in the figure. When the screen n n fits 
snugly it remains in place of its own accord, or it may be wired to 
the gauze covering m m. The boiling-tube a a and the burner r r being 
each supported by an ordinary retort holder with universal clamps, are 
easily adjustable at pleasure. 
In the case of substances which are spontaneously inflammable at 
their boiling points, like sulphur, the oxygen of the tube is so soon ex- 
hausted that ebullition takes place without interruption. Hence the 
introduction of special gases like N 2 or 0O 2 is rarely necessary; a great 
advantage, inasmuch as the introduction of gas, no matter how slowly 
or how regularly, always interferes with the constancy of temperature. 
In case of mercury, however, the metal must be renewed from time to 
(740) 
