product. Theoretically HBr and HI might be made in the same way
from NaBr and NaI, but the affinity of H for Br and I is weak,
and the acids separate into their elements, when thus prepared.

83. To make HI.

Experiment 50.--Drop into a t.t. three or four crystals of I, and
add 10 cc. H2O. Hold in the water the end of a d.t. from which
H2S gas is escaping. Observe any deposit, and write the reaction.

FLUORHYDRIC ACID.

84. Preparation and Action.

Experiment 51.--Put 3 or 4 g. powdered CaF2, i.e. fluor spar or
fluorite, into a shallow lead tray, e.g. 4x5 cm, and pour over it
4 or 5 cc. H2SO4. A piece of glass large enough to cover this
should previously be warmed and covered on one side with a very
thin coat of beeswax. To distribute itevenly, warm the other side
of the glass over a flame. When cool, scratch a design (Fig. 24)
through the wax with a sharp metallic point. Lay the glass, film
side down, over the lead tray. Warm this five minutes or more by
placing it high over a small flame (Fig. 25) to avoid melting the
wax. Do not inhale the fumes. Take away the lamp, and leave the
tray and glass where it is not cold, for half an hour or more.
Then remove the wax and clean the glass with naphtha or benzine.
Look for the etching.

Two things should have occurred: (1) the generation of HF. Write
the equation for it. (2) Its etching action on glass. In this