[Footnote 1:  Comptes Rendus, vol. cii., p. 1543, and Chemical
  News, vol. liv., p. 36.]

To obtain pure anhydrous hydrofluoric acid, we begin by preparing fluorhydrate of fluoride of potassium, taking all the precautions pointed out by M. Fremy.  When the salt is obtained pure, it is dried on a water bath at 100 deg., and the platinum capsule containing it is then placed in a vacuum in the presence of concentrated sulphuric acid, and two or three sticks of potash fused in a silver crucible.  The acid and potash are renewed every morning for a fortnight, and the vacuum is kept at 2 cm. of mercury.  Care must be taken during this desiccation to pulverize the salt every day in an iron mortar, so as to renew the surface.  When the fluorhydrate contains no more water it falls to powder, and is then fit to serve for the preparation of fluoric acid; the fluorhydrate of fluoride of potassium, if well prepared, is much less deliquescent than the fluoride.

When the fluoride is quite dry, it is quickly introduced into a platinum alembic, which has just been dried by heating it to redness.  The whole is kept at a gentle temperature for an hour or an hour and a half, so as to allow the decomposition to commence very slowly; the first portions of acid which come over are rejected as they carry with them traces of water remaining in the salt.  The platinum receiver is then attached, and the heat increased, allowing the decomposition to proceed with a certain degree of slowness.  The receiver is then surrounded with a mixture of ice and salt, and from this moment all the hydrofluoric acid is condensed as a limpid liquid, boiling at 19.5 deg., very hygroscopic, and, as is well known, giving abundant fumes in presence of the atmospheric moisture.

During this operation the platinum U tube, dried with the greatest care, has been fixed with a cork in a cylindrical glass vessel surrounded with chloride of methyl.  Up to the moment of introducing the hydrofluoric acid, the leading tubes are attached to drying tubes containing fused caustic potash.  To introduce the hydrochloric acid into the apparatus, it may be absorbed through one of the lateral tubes in the receiver in which it is condensed.

In some experiments we have directly condensed the hydrofluoric acid in the U tube surrounded with chloride of methyl; but in this case care must be taken that the tubes are not clogged up by small quantities of fluoride carried over, which would infallibly lead to an explosion and projections, which are always dangerous with so corrosive a liquid.

When we have introduced in advance in the small platinum apparatus a determined amount of hydrofluoric acid cooled with chloride of methyl, in tranquil ebullition at a temperature of -23 deg., the current of 20 cells of Bunsen large size, arranged in series, is passed through by means of the electrodes.  An amperemeter in the circuit admits of the intensity of the current being observed.