The boron was prepared by the thermal decomposition
of diborane by pa sing diboran e diluted with
helium through a quartz tube heated to 6000 C [1]. The amorphous boron was removed from the tube,
placed in a tantalum combustion boat, and heated
for 1 hI' at about 7000 C at a pressure of Ie s than
1 X 10- 5 mm of mercury. Spectrochemical examination
of the boron by the Spectrochemistry Sec tion
of the Bureau showed only negligible Ll'accs of metal
impurities. The effect of a small amount of hydrogen
remainin g in the boron was calculated Lo be less than
the estimated un certainty of Lhc measuremen ts.
The chlorine, obLained from Lhe MaLheson Com- pan~', was about 99 percent pure ; Lh e chief impurity
was 0.9 percenL of carbon dioxide. It was dried by
passing over phosphorus pentoxidc. The helium was
pUl'ifi.ed by heatin g it to 6000 C in a copper-oxide
furnace alld directing it successively Lhl'ough Ascarite, anhydrous magnesium perchlorate, and phosphorus
pentoxide. A section of the calorimetric vessel is shown in
figure 1. It consisted of a quartz tub e .fl closed at
the bottom and fiLted with a manganin h eating coil
B surrounded by a silver shield G and a vacuum
jacket D. The boron sample was placed in the
quartz crucible E , and the chlo)'in e introduced
through the quartz tube F. Th e exit gases passed
through tb e glass helix 0, in which they weTe cooled
to calorime ter temperature. The calorimeter was of the isothermal jacket typ e
described in a previous paper [2]. Calorimeter tem- peratures were mea sUl'ed by means of a pIa tinum
resistance thermometer in conjunction with a G- 2
Mueller bridge and a galvanometer of high sensitivity.
Readings on the galvanometer scale wer e
made to Lhe nearest 0.5 mm which was equivalent to
50 J.Ldeg C. The electrical energy was obtained from
a 120-v storage battery from which no other curren t
was drawn dming these experiments. The quantity